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

gio/tests/gdbus-connection.c

@@ -141,7 +141,7 @@ test_connection_life_cycle (void)
    *
    */
   c = g_bus_get_sync (G_BUS_TYPE_SESSION, NULL, &error);
-  _g_assert_error_domain (error, G_IO_ERROR);
+  g_assert (error != NULL);
   g_assert (!g_dbus_error_is_remote_error (error));
   g_assert (c == NULL);
   g_error_free (error);