glib-unix: New API to watch some Unix signals

glib-unix: New API to watch some Unix signals

This new API allows watching a few select Unix signals;
looking through the list on my system, I didn't see anything
else that I think it'd reasonable to watch.

We build on the previous patch to make the child watch helper thread
that existed on Unix handle these signals in the threaded case.
In the non-threaded case, they're just global variables.

https://bugzilla.gnome.org/show_bug.cgi?id=644941