Improve compliance with RFC 4895 and RFC 6458.

Improve compliance with RFC 4895 and RFC 6458.

Silently dicard SCTP chunks which have been requested to be
authenticated but are received unauthenticated no matter if support
for SCTP authentication has been negotiated. This improves compliance
with RFC 4895.

When the application uses the SCTP_AUTH_CHUNK socket option to
request a chunk to be received in an authenticated way, enable
the SCTP authentication extension for the end-point. This improves
compliance with RFC 6458.

Discussed with:	Peter Lei
MFC after:	3 days