Port to g_unix_signal_add(), drop GdmSignalHandler

The level of copy/paste going on here before is rather astonishing.
For example, in some cases, I dropped spurious handling of SIGHUP,
when the code didn't have any settings to reread.

Anyways, the code is now clearer, and we get to drop all the bits of
gdm-signal-handler.[ch] for the integrated GLib handling.

https://bugzilla.gnome.org/show_bug.cgi?id=676181