Rework detection / enabling of SCTP support.

Rather than doing checks for platforms that we believe support SCTP,
instead look for an indication (notably the presence of <netinet/sctp.h>)
that it's supported.  This makes the conditionals for SCTP more obvious.
In addition, it opens up the possibility that SCTP might work on some
new OS that's not FreeBSD or Linux.

This change may force some additional build-time requirements on Linux,
such as lksctp-tools-devel on CentOS / Fedora or libsctp-dev on
Ubuntu.

Committing this first cut for review and to enable testing on multiple
platforms.  So far this works correctly on Linux (SCTP support) and
MacOS (no SCTP support).