MINOR: log: adds syslog udp message handler and parsing.

MINOR: log: adds syslog udp message handler and parsing.

This patch introduce a new fd handler used to parse syslog
message on udp.

The parsing function returns level, facility and metadata that
can be immediatly reused to forward message to a log server.

This handler is enabled on udp listeners if proxy is internally set
to mode PR_MODE_SYSLOG