Add a --sctp flag to nc.

The change adds an SCTP mode akin to UDP mode. This is handy for doing
smoke testing of SCTP.

Use a long option to minimize the risk of option conflicts with OpenBSD.
For bonus points, this change unbreaks --no-tcpopt by adding a missing
case required by getopt_long().

Reviewed by: delphij, tuexen
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D25610