[SCTP] Rename SCTP specific control message flags.

Rename SCTP specific control message flags to use SCTP_ prefix rather than
MSG_ prefix as per the latest sctp sockets API draft.

Signed-off-by: Ivan Skytte Jorgensen <isj-sctp@i1.dk>
Signed-off-by: Sridhar Samudrala <sri@us.ibm.com>