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.c

@@ -17,9 +17,7 @@
 **
 */
 
-#if HAVE_CONFIG_H
 #include "config.h"
-#endif /*HAVE_CONFIG_H*/
 
 #include <glib.h>
 #include <string.h>		/* memset */