Add newsyslog capability to write RFC5424 compliant rotation message.

Add newsyslog capability to write RFC5424 compliant rotation message.

This modification adds the capability to newsyslog to write the
rotation message in a format that is compliant with RFC5424. This
capability is enabled on a per-log file basis through a new value
("T") in the flags field in newsyslog.conf. This is useful on systems
that use the RFC5424 format for log files so that the rotation message
format matches that of the other log messages. There has been recent
mention of adding an RFC5424 compliant mode to syslogd and at least
one alternative system log daemon (rsyslogd) that already has the
capability to use that format.

Reviewed by:	vangyzen, ngie
Approved by:	vangyzen (mentor)
MFC after:	2 months
Relnotes:	yes
Sponsored by:	Dell EMC
Differential Revision:	https://reviews.freebsd.org/D10253