sctp: update manpages

Add missing documentation for sctp_sendv(3) and sctp_recvv(3).

Add a note that sctp_send(3), sctp_sendx(3) and sctp_recvmsg(3) are
deprecated by RFC 6458.

Add a STANDARDS section to all functions specified in RFC 6458 to
indicate their standards conformance.

MFC after: 2 weeks
Reviewed by: imp, bcr, kp, tuexen
Pull Request: https://github.com/freebsd/freebsd-src/pull/965