netdev implementation for FreeBSD

netdev implementation for FreeBSD

This patch adds new netdev classes that implement
"system" and "tap" devices on FreeBSD using the
libpcap library. This enables the use of the
"netdev" datapath_type of Open vSwitch on FreeBSD.

Signed-off-by: Gaetano Catalli <gaetano.catalli@gmail.com>
Signed-off-by: Ed Maste <emaste@adaranet.com>
Signed-off-by: Giuseppe Lettieri <g.lettieri@iet.unipi.it>
Signed-off-by: Ben Pfaff <blp@nicira.com>