The segfault looks like this:
```
Thread 3 Crashed:
0   com.docker.vpnkit             	       0x10231b8b8 camlLuv__Helpers__set_reference_inner_1334 + 40
1   com.docker.vpnkit             	       0x10231b8b8 camlLuv__Helpers__set_reference_inner_1334 + 40
2   com.docker.vpnkit             	       0x1029c1108 caml_callback_exn + 24 (callback.c:111) [inlined]
3   com.docker.vpnkit             	       0x1029c1108 caml_callback + 44 (callback.c:165)
4   com.docker.vpnkit             	       0x102980aa0 luv_async_trampoline + 44 (helpers.c:64)
5   com.docker.vpnkit             	       0x102986a98 uv__async_io + 308 (async.c:163)
6   com.docker.vpnkit             	       0x102996be8 uv__io_poll + 992
7   com.docker.vpnkit             	       0x102986ec4 uv_run + 372 (core.c:389)
8   com.docker.vpnkit             	       0x10297cdd4 luv_stub_blocking_1_uv_run + 104 (c_generated_functions.c:1800)
9   com.docker.vpnkit             	       0x1029cc4e8 caml_c_call + 28
10  com.docker.vpnkit             	       0x1029c0e60 caml_callback_exn + 44 (callback.c:111)
11  com.docker.vpnkit             	       0x1029984a8 caml_thread_start + 112 (st_stubs.c:548)
12  libsystem_pthread.dylib       	       0x180a7d06c _pthread_start + 148
13  libsystem_pthread.dylib       	       0x180a77e2c thread_start + 8
```

I reproduced this by `docker pull`ing an image, and using Control+C. The proxy
is interrupted and closes the stream, while there is still a background read/write
loop in progress. The assumption was that read after close would return a harmless
error, but this isn't guaranteed by luv. Therefore we catch this case and return
an error ourselves.

Signed-off-by: David Scott <dave@recoil.org>