Be more compliant with RFC 4447.

Be more compliant with RFC 4447.

When sending a label withdraw during the pseudowire Control Word
negotiation, append a "Wrong C-bit" status TLV after the FEC TLV (in
conformance to RFC 4447 section 6.2). Apparently this has no use other
than aiding in troubleshooting.

Also, extend the recv_labelmessage() function to accept Status TLVs and
ignore them instead of shutting down the session.