ofproto-dpif: Enable smooth transition between CFM and BFD.

ofproto-dpif: Enable smooth transition between CFM and BFD.

When user switches between using CFM and BFD, there will be a short
down time before the new protocol goes up.  This can unintentionally
change the traffic pattern of the bundled ports.  To prevent this,
it is proposed that user can enable both CFM and BFD before transition,
wait for the new protocol to go up, and then disable the old protocol.

To make this scheme work, this commit modifies the port_run() in
ofproto-dpif.c, so that when both CFM and BFD are used, if either shows
correct status, the port is considered usable in the bundle.

Signed-off-by: Alex Wang <alexw@nicira.com>