ofproto-dpif-xlate: Support BFD, CFM, carrier, and LACP for port liveness.

This is simpler and shorter than handling each of these by itself.

CC: Niels van Adrichem <N.L.M.vanAdrichem@tudelft.nl>
Suggested-by: Alex Wang <alexw@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Alex Wang <alexw@nicira.com>