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
GLib historically has been designed to be "mostly" portable; there
are some functions only available on Unix like g_io_channel_unix_new(),
but these are typically paired with obvious counterparts for Win32.
However, as GLib is used not only by portable software, but components
targeting Unix (or even just Linux), there are a few cases where it
would be very convenient if GLib shipped built-in functionality.
This initial patch is a basic wrapper around pipe2(), including
fallbacks for older kernels. This pairs well with the
existing g_spawn_*() API and its child_setup functionality.
However, in the future, I want to add a signal() wrapper here,
complete with proxying the signal to a mainloop. I have initial code
for this, but doing it sanely (including factoring out gmain.c's
private worker thread), is a complex task, and I don't want to block
on that.
See also gwin32.h for Win32 specific functionality.
https://bugzilla.gnome.org/show_bug.cgi?id=644941
