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.
Until now, mu would _not_ follow symlinks; with these changes, we do.
There were some complications with that ~10 years ago, but I forgot the
details. So let's re-enable. At least one thing is in place now: moving
between file systems.
Fixes#1489Fixes#1628 (technically, this came with slightly earlier commit)