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.
Instead of using ~/.mu, use the XDG Base Directory Specification, typically:
~/.cache/xapian
~/.cache/mu.log
~/.cache/parts
~/.config/bookmarks
Update dependencies, documentation.