Add g_spawn_check_exit_status()

Many (if not "almost all") programs that spawn other programs via
g_spawn_sync() or the like simply want to check whether or not the
child exited successfully, but doing so requires use of
platform-specific functionality and there's actually a fair amount of
boilerplate involved.

This new API will help drain a *lot* of mostly duplicated code in
GNOME, from gnome-session to gdm.  And we can see that some bits even
inside GLib were doing it wrong; for example checking the exit status
on Unix, but ignoring it on Windows.

https://bugzilla.gnome.org/show_bug.cgi?id=679691

docs/reference/glib/glib-sections.txt

@@ -1093,6 +1093,7 @@ GSpawnChildSetupFunc
 g_spawn_async_with_pipes
 g_spawn_async
 g_spawn_sync
+g_spawn_check_exit_status
 g_spawn_command_line_async
 g_spawn_command_line_sync
 g_spawn_close_pid