log to journal

[lnussel]

snapper uses it's own log file in /var/log/snapper.log. Nowadays system components are expected to log to the journal. That's not only more convenient to query logs of individual services but also allows to turn off logging to disk on systems where write operations must be limited.
At the same time the default log level should probably be reduced from milestone to warning.

Can be done via either the native api such as sd_journal_print or by just writing to a file descriptor

[lnussel]

while investigating #866 having all info in the journal would have been convenient indeed