logger: add rfc5424 support

Add support the more recent syslog protocol and make it default.  The
older BSD syslog protocol can still be used with option --rfc3164.
Protocols are meaningful only when messages are sent to remote syslog
server.

Requested-by: Kodiak Firesmith <ksf@sei.cmu.edu>
Signed-off-by: Sami Kerola <kerolasa@iki.fi>