MEDIUM: logs: add support for RFC5424 header format per logger

MEDIUM: logs: add support for RFC5424 header format per logger

The function __send_log() iterates over senders and passes the header as
the first vector to sendmsg(), thus it can send a logger-specific header
in each message.

A new logger arguments "format rfc5424" should be used in order to enable
RFC5424 header format. For example:

    log 10.2.3.4:1234 len 2048 format rfc5424 local2 info