MFC r332353:

Don't show the number of currently established SCTP associations,
since this is not monotonically increasing. It's number can be
derived from the other counters shown.

MFC r326672:

Retire SCTP_WITH_NO_CSUM option.

This option was used in the early days to allow performance measurements
extrapolating the use of SCTP checksum offloading. Since this feature
is now available, get rid of this option.
This also un-breaks the LINT kernel. Thanks to markj@ for making me
aware of the problem.

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Add support to systat to display SCTP statistics.

MFC after: 1 week