When debugging homed while being logged into a user account manged by
homed it is a good idea to be able to run a second copy of homed. In
order to not collide with its AF_UNIX socket and bus name use, let's add
a new env var $SYSTEMD_HOME_DEBUG_SUFFIX, when set the busnames/socket
names are suffixed by it. When setting this while debugging one can
invoke an additional copy without interfering with the host one.

When exiting, let's explicitly wait for our worker processes to finish
first. That's useful if unmounting of /home/ is scheduled to happen
right after homed is down, as we then can be sure that the home
directories are properly unmounted and detached by the time homed is
fully terminated (otherwise it might happen that our worker gets killed
by the service manager, thus leaving the home directory and its backing
devices up/left for auto-clean which might be async).

Likely fixes systemd#16842

…ck-device is not supported

The directory backend needs a file system path, and not a raw block
device. That's only supported for the LUKS2 backend.

Let's make this clearer in the man page and also generate a better error
message if attempted anyway.

Fixes: systemd#17068

The ext4 fs resize ioctl needs CAP_SYS_RESOURCE, irritatingly. Let's
grant it to homed hence.

Fixes: systemd#15115