packetdrill: add initial support for SCTP socket options.

This commit adds support for the following SCTP socket options:
* SCTP_RTOINFO
* SCTP_INITMSG
* SCTP_NODELAY
* SCTP_MAXSEG
* SCTP_DELAYED_SACK
* SCTP_MAX_BURST

Tested on Linux and FreeBSD. NetBSD and OpenBSD doen't support
SCTP and should not be affected.

Signed-off-by: Michael Tuexen <tuexen@fh-muenster.de>