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.

NEWS.org

@@ -5,6 +5,9 @@
 
 *** mu
 
+    - Where available (and with suitable glib), log to the systemd journal
+      instead of a =~/.cache/mu.log=
+
     - Follow symlinks in maildirs, and support moving messsages between across
       multiple filesystems (but note that that is quite a bit slower than the
       single-filesystem case)