vlog: abstract out interface to syslog daemon

vlog: abstract out interface to syslog daemon

This patch helps to address two issues that are present on Ubuntu
15.04 (and most likely other Linux distributions) where rsyslog daemon
is configured to relay log messages from OVS to a remote log collector
and syslog format being used is something other than the one defined in
RFC 3164.  These two issues are:

1. libc syslog() function always adds RFC 3164 prefix to syslog
   messages before sending them over /dev/log Unix domain socket.
   This does not allow us to use libc syslog() function to log in
   RFC 5424 format;  and

2. rsyslogd daemon that comes with Ubuntu 15.04 is too old and
   uses hardcoded syslog message parser when it received messages
   over /dev/log UNIX domain socket.

Solution to those two issues would be to use the newly introduced
--syslog-method=udp:127.0.0.1:514 command line argument when starting
OVS.

Signed-off-by: Ansis Atteka <aatteka@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>