sys-logger: Optionally support mapping strongSwan loglevels to syslog levels

sys-logger: Optionally support mapping strongSwan loglevels to syslog levels

strongSwan logs all syslog messages using LOG_INFO for historical reasons,
regardless of the strongSwan loglevel used producing the log message.

In some setups with advanced logging infrastructure, it may be feasible
to be more verbose when logging in strongSwan, but then filter messages
on the syslog server. While this may be possible by custom syslog filtering
rules matching the log level included with the log_level setting, this is
not super convenient.

So add a new map_level setting, which can map strongSwan loglevels to
syslog loglevels. By default this is disabled, keeping the existing
behavior. If enabled, it maps strongSwan loglevels to syslog loglevels
at a given syslog loglevel offset.

Closes strongswan/strongswan#859