Let syslog(3) use RFC 5424.

With r332099 changing syslogd(8) to parse RFC 5424 formatted syslog
messages, go ahead and also change the syslog(3) libc function to
generate them. Compared to RFC 3164, RFC 5424 has various advantages,
such as sub-second precision for log entry timestamps.

As this change could have adverse effects when not updating syslogd(8)
or using a different system logging daemon, add a notice to UPDATING and
increase __FreeBSD_version.

Differential Revision: https://reviews.freebsd.org/D14926