Restore local kernel "prog" filtering lost in r332099.

This behavior is most relevant for ipfw(4) as documented in syslog.conf(5).
The recent addition of property-based regex filters in r359327 is a
fine workaround for this but the behavior was present since 1997 and
documented.

This only fixes local matching of the "kernel program". It does not
change the forwarded format at all. On the remote side it will still
be "kernel: ipfw:" and not be parsed as a kernel message. This matches
old behavior.

MFC after: 2 weeks
Reviewed by: markj
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D24286