When NM is restarted and a new object-manager is created, ensure that
signal handlers are disconnected from the old one.

Fixes the following:
  assertion failed: (object_manager == priv->object_manager)

 #0  __GI_raise (sig=sig@entry=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:56
 #1  __GI_abort () at abort.c:90
 #2  g_assertion_message (domain=domain@entry=0x7fcac0b845ff "libnm", file=file@entry=0x7fcac0b84c95 "libnm/nm-client.c", line=line@entry=2506, func=func@entry=0x7fcac0b863a0 <__func__.34881> "name_owner_changed", message=message@entry=0x7fcac20b05f0 "assertion failed: (object_manager == priv->object_manager)") at gtestutils.c:2429
 #3  g_assertion_message_expr (domain=domain@entry=0x7fcac0b845ff "libnm", file=file@entry=0x7fcac0b84c95 "libnm/nm-client.c", line=line@entry=2506, func=func@entry=0x7fcac0b863a0 <__func__.34881> "name_owner_changed", expr=expr@entry=0x7fcac0b856a0 "object_manager == priv->object_manager") at gtestutils.c:2444
 #4  name_owner_changed (object=<optimized out>, pspec=<optimized out>, user_data=0x7fcac204e480) at libnm/nm-client.c:2506
 #8  <emit signal notify:name-owner on instance 0x7fcac2053c60 [GDBusObjectManagerClient]> (instance=instance@entry=0x7fcac2053c60, signal_id=<optimized out>, detail=<optimized out>) at gsignal.c:3439
     #5  g_closure_invoke (closure=0x7fcac20af390, return_value=return_value@entry=0x0, n_param_values=2, param_values=param_values@entry=0x7ffde58d9ec0, invocation_hint=invocation_hint@entry=0x7ffde58d9e60) at gclosure.c:801
     #6  signal_emit_unlocked_R (node=node@entry=0x7fcac2052090, detail=detail@entry=185, instance=instance@entry=0x7fcac2053c60, emission_return=emission_return@entry=0x0, instance_and_params=instance_and_params@entry=0x7ffde58d9ec0) at gsignal.c:3627
     #7  g_signal_emit_valist (instance=<optimized out>, signal_id=<optimized out>, detail=<optimized out>, var_args=var_args@entry=0x7ffde58da050) at gsignal.c:3383
 #9  g_object_dispatch_properties_changed (object=0x7fcac2053c60 [GDBusObjectManagerClient], n_pspecs=<optimized out>, pspecs=<optimized out>) at gobject.c:1061
 #10 g_object_notify (pspec=<optimized out>, object=0x7fcac2053c60 [GDBusObjectManagerClient]) at gobject.c:1155
 #11 g_object_notify (object=object@entry=0x7fcac2053c60 [GDBusObjectManagerClient], property_name=property_name@entry=0x7fcabe9d2b29 "name-owner") at gobject.c:1202
 #12 on_notify_g_name_owner (object=<optimized out>, pspec=<optimized out>, user_data=0x7fcac2053c60) at gdbusobjectmanagerclient.c:1262
 #16 <emit signal notify:g-name-owner on instance 0x7fcaa8004440 [GDBusProxy]> (instance=instance@entry=0x7fcaa8004440, signal_id=<optimized out>, detail=<optimized out>) at gsignal.c:3439
     #13 g_closure_invoke (closure=0x7fcaa80194f0, return_value=return_value@entry=0x0, n_param_values=2, param_values=param_values@entry=0x7ffde58da370, invocation_hint=invocation_hint@entry=0x7ffde58da310) at gclosure.c:801
     #14 signal_emit_unlocked_R (node=node@entry=0x7fcac2052090, detail=detail@entry=299, instance=instance@entry=0x7fcaa8004440, emission_return=emission_return@entry=0x0, instance_and_params=instance_and_params@entry=0x7ffde58da370) at gsignal.c:3627
     #15 g_signal_emit_valist (instance=<optimized out>, signal_id=<optimized out>, detail=<optimized out>, var_args=var_args@entry=0x7ffde58da500) at gsignal.c:3383
 #17 g_object_dispatch_properties_changed (object=0x7fcaa8004440 [GDBusProxy], n_pspecs=<optimized out>, pspecs=<optimized out>) at gobject.c:1061
 #18 g_object_notify (pspec=<optimized out>, object=0x7fcaa8004440 [GDBusProxy]) at gobject.c:1155
 #19 g_object_notify (object=object@entry=0x7fcaa8004440 [GDBusProxy], property_name=property_name@entry=0x7fcabe9d2b27 "g-name-owner") at gobject.c:1202
 #20 on_name_owner_changed (connection=<optimized out>, sender_name=<optimized out>, object_path=<optimized out>, interface_name=<optimized out>, signal_name=<optimized out>, parameters=<optimized out>, user_data=0x7fcaa8015b50) at gdbusproxy.c:1353
 #21 emit_signal_instance_in_idle_cb (data=0x7fcaa40ff400) at gdbusconnection.c:3701
 #22 g_main_context_dispatch (context=0x7fcac204eea0) at gmain.c:3152
 #23 g_main_context_dispatch (context=context@entry=0x7fcac204eea0) at gmain.c:3767
 #24 g_main_context_iterate (context=context@entry=0x7fcac204eea0, block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>) at gmain.c:3838
 #25 g_main_context_iteration (context=0x7fcac204eea0, context@entry=0x0, may_block=may_block@entry=1) at gmain.c:3899
 #26 nmc_readline_helper (prompt=prompt@entry=0x7fcac2087f60 "The connection is not saved. Do you really want to quit? (yes/no) [no] ") at clients/cli/common.c:986
 #27 nmc_readline (prompt_fmt=<optimized out>) at clients/cli/common.c:1055
 #28 confirm_quit () at clients/cli/connections.c:6459
 #29 do_connection_edit (connection_type=<optimized out>, connection=0x7fcac208eca0, nmc=0x7fcac12a3a60 <nm_cli>) at clients/cli/connections.c:7611
 #30 do_connection_edit (nmc=0x7fcac12a3a60 <nm_cli>, argc=1, argv=0x7ffde58db0b0) at clients/cli/connections.c:7948
 #31 call_cmd (nmc=0x7fcac12a3a60 <nm_cli>, simple=0x7fcac2052490 [GSimpleAsyncResult], cmd=0x7fcac1291ae0 <connection_cmds+128>, argc=2, argv=0x7ffde58db0a8) at clients/cli/common.c:1315
 #32 got_client (source_object=<optimized out>, res=<optimized out>, user_data=0x7fcac2051830) at clients/cli/common.c:1297
 #33 g_simple_async_result_complete (simple=0x7fcac2052500 [GSimpleAsyncResult]) at gsimpleasyncresult.c:801
 #34 client_inited (source=0x7fcac204e480 [NMClient], result=0x7fcac20565f0, user_data=0x7fcac2052500) at libnm/nm-client.c:1839
 #35 g_simple_async_result_complete (simple=0x7fcac20565f0 [GSimpleAsyncResult]) at gsimpleasyncresult.c:801
 #36 init_async_complete (init_data=init_data@entry=0x7fcac2046820) at libnm/nm-client.c:2339
 #37 async_inited_obj_nm (init_data=0x7fcac2046820) at libnm/nm-client.c:2337
 #38 async_inited_obj_nm (object=0x7fcac2073080 [NMRemoteConnection], result=0x7fcac2089ed0, user_data=0x7fcac2046820) at libnm/nm-client.c:2357
 #39 g_simple_async_result_complete (simple=0x7fcac2089ed0 [GSimpleAsyncResult]) at gsimpleasyncresult.c:801
 #40 init_async_parent_inited (error=0x0, init_data=0x7fcaa408a0f0) at libnm/nm-remote-connection.c:677
 #41 init_async_parent_inited (source=<optimized out>, result=<optimized out>, user_data=0x7fcaa408a0f0) at libnm/nm-remote-connection.c:689
 #42 g_simple_async_result_complete (simple=0x7fcac2089c30 [GSimpleAsyncResult]) at gsimpleasyncresult.c:801
 #43 complete_in_idle_cb (data=<optimized out>) at gsimpleasyncresult.c:813
 #44 g_main_context_dispatch (context=0x7fcac204eea0) at gmain.c:3152
 #45 g_main_context_dispatch (context=context@entry=0x7fcac204eea0) at gmain.c:3767
 #46 g_main_context_iterate (context=0x7fcac204eea0, block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>) at gmain.c:3838
 #47 g_main_loop_run (loop=0x7fcac2045ab0) at gmain.c:4032
 #48 main (argc=<optimized out>, argv=<optimized out>) at clients/cli/nmcli.c:642

https://bugzilla.redhat.com/show_bug.cgi?id=1471245
(cherry picked from commit 7758071)