server: Process SIGUSR1 more carefully

If the slave is removed as a separate process, it means that we need
to be more careful with our handling of SIGUSR1. If multiple X servers
are launched at once, we need to use siginfo_t to get the PID of the
thing that sent the user signal, and make sure we signal the correct
GdmServer.

glib doesn't have native siginfo support, so do it ourselves by using
a worker thread that spins around waiting for sigwaitinfo.

https://bugzilla.gnome.org/show_bug.cgi?id=724382