pf: support 'return' for SCTP

Send an SCTP Abort message if we're refusing a connection, just like we
send a RST for TCP.

MFC after: 3 weeks
Sponsored by: Orange Business Services
Differential Revision: https://reviews.freebsd.org/D40864

(cherry picked from commit d1bc1e9e1ae04016e16154884914d839566ebaec)