bus signal_handler: call _exit in the unlikely event that the pipe is full or invalid

bus signal_handler: call _exit in the unlikely event that the pipe is full or invalid

On OSs with abstract sockets, this is close enough. On OSs without
abstract sockets, this results in failing to clean up Unix sockets
in /tmp if someone has sent us thousands of SIGHUP signals since we
last entered the main loop - I think that's acceptable.

The reload pipe should never get closed, but if it is for some reason,
we want a SIGTERM after that to cause an exit too.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=38656
Reviewed-by: Will Thompson <will.thompson@collabora.co.uk>