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.

mu/mu-config.h

@@ -97,8 +97,8 @@ struct _MuConfig {
 	char		*cmdstr;        /* cmd string, for user
 					 * info */
 	/* general options */
-	gboolean	 quiet;	/* don't give any output */
-	gboolean	 debug;	/* spew out debug info */
+	gboolean	 quiet;	        /* don't give any output */
+	gboolean	 debug;	        /* log debug-level info */
 	gchar		*muhome;	/* the House of Mu */
 	gboolean	 version;	/* request mu version */
 	gboolean	 log_stderr;	/* log to stderr (not logfile) */