netdev: Abstract "features" interface away from OpenFlow 1.0.

netdev_get_features() and other functions have always used OpenFlow 1.0
"enum ofp_port_features" bits as part of their interface.  This commit
switches over to using an internally defined interface that is not tied
directly to any OpenFlow version, making evolution of each side of the
interface easier in the future.

Reviewed-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>