Allow caller to disable logging on remote systems 2.0

This commit introduces a new possible value for the syslog_facility
configuration option, namely 'off' (or '0', 'no', 'false'…).

If the syslog_facility is set to any of those values, then the log
invocation will return without logging.

Signed-off-by: martin f. krafft <madduck@madduck.net>