Note this option only has a long option flag; we're running out of
letters for short options.

Based heavily on a patch submitted in Issue 131 (SCTP support for
iperf); I added support for FreeBSD and did some other packaging and
documentation improvements.

We probably shouldn't tie SCTP support to looking specifically for
Linux or FreeBSD; we probably leave support enabled all the time if
possible, possibly with some configure-time checks.