periodic: Support RFC 5424 syslog timestamps

This is based on an initial implementation by michaelo in
https://reviews.freebsd.org/D54361.

PR: 270497
Reported by: michaelo
Reviewed by: michaelo
Tested by: michaelo
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D54606