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>
docs: add logger protocol notes to manual page

Inform about rfc5424 support, how to control what it includes to
submission, and that it is the new default when sending syslog messages
to remote server without specifying protocol.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
logger: add process --id=parent optional argument

When scripts send several messages they will be easier to group together
when parent process id is printed rather than id of the each logger
process.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>