syslogd: Add rfc3164-strict format option

The syslogd bsd/rfc3164 message format does not strictly conform to the
format recommended in RFC 3164 when syslog messages are forwarded from
remote hosts.

The new rfc3164-strict format generates messages that conform with the
RFC 3164 recommended format. The existing format is maintained for
backwards compatibility.

PR: 220246
Reviewed by: markj
MFC after: 3 weeks
Pull Request: https://github.com/freebsd/freebsd-src/pull/1536