rework logging system

reimplement the old mu-log.[ch] into mu-logging.{cc,hh}

If available (and using an appropriately equipped glib), log to the
systemd journal

Only g_criticals have stderr output, all the other g_* go to the log
file / journal.

guile/mu-guile.c

@@ -86,7 +86,7 @@ mu_guile_init_instance (const char *muhome)
 
 	setlocale (LC_ALL, "");
 
-	if (!mu_runtime_init (muhome, "guile"))
+	if (!mu_runtime_init (muhome, "guile", FALSE))
 		return FALSE;
 
 	err = NULL;