ldpd: Relay data plane pseudowire status in LDP notification
Provide a way for the data plane to indicate pseudowire
status (such as: not forwarding, AC failure).
On a data plane pseudowire install failure, data plane
sets the pseudowire status.
Zebra relays the pseudowire status to LDP.
LDP includes the pseudowire status in the LDP notification
to the LDP peer.