Introduce syslog service for Casper.

syslog in libc secretly reconnects to the daemon.
Another issue is that we don't have any information from openlog(3) if we
succeeded to open log or not so we don't know if we are ready
to enter cabability mode.
Because all of that we decided we need a syslog service for Caspser.

Reviewed by: bapt@
Differential Revision: https://reviews.freebsd.org/D12824