minor, support logging to syslog via ud_openlog(NULL);

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
minor, adapt ud_logout to printing to syslog

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
Merge branch 'core/logger' into next

* core/logger:
  minor, adapt ud_logout to printing to syslog
  minor, support logging to syslog via ud_openlog(NULL);