lib: RFC5424 & journald extended syslog target

lib: RFC5424 & journald extended syslog target

Not much to say here, user docs are coming up in a separate commit.
RFC5424 and (systemd's) journald allow passing structured key-value
data.  This stuffs the metadata we have available into there.

The "does the system syslogd support RFC5424" question is unfortunately
not easily answered, so we can only give an affirmative answer on NetBSD
5.0+ or FreeBSD 12+.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>