When adding support for sending SCTP packets containing an ABORT chunk
to ipfw in https://svnweb.freebsd.org/changeset/base/326233,
a dependency on the SCTP stack was added to ipfw by accident.

This was noted by Kevel Bowling in https://reviews.freebsd.org/D13594
where also a solution was suggested. This patch is based on Kevin's
suggestion, but implements the required SCTP checksum computation
without any dependency on other SCTP sources.

While there, do some cleanups and improve comments.

Thanks to Kevin Kevin Browling for reporting the issue and suggesting
a fix.