core: use g_unix_signal_add() for more reliable Unix signal handling

There were a few problems with MM's existing signal handling, first
of which was that calling g_main_loop_quit() from a signal handler
only works 50% of the time due to severe restrictions on what you
can do from the handler.  This caused INT or TERM to sometimes be
ignored by MM.

Instead, use the glib signal functions which ensure that the handler
is run in the right context, where we can do anything we want.

src/mm-log.c

@@ -246,11 +246,6 @@ mm_log_setup (const char *level,
     return TRUE;
 }
 
-void
-mm_log_usr1 (void)
-{
-}
-
 void
 mm_log_shutdown (void)
 {