syslogd: Allow killing when in foreground

Normally, syslogd reacts only to SIGTERM, and ignores SIGINT and SIGQUIT
unless in debug mode. Extend that to also apply when running in the
foreground. Take this opportunity to comment the event loop.

MFC after: 1 week
Reviewed by: jfree
Differential Revision: https://reviews.freebsd.org/D55886