bfd: Implement forwarding_if_rx.

bfd: Implement forwarding_if_rx.

This commit adds a new boolean option "forwarding_if_rx" to bfd.

When forwarding_if_rx is true the interface will be considered
capable of packet I/O as long as there is packet received at
interface.  This is important in that when link becomes temporarily
conjested, consecutive BFD control packets can be lost.  And the
forwarding_if_rx can prevent link failover by detecting non-control
packets received at interface.

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