Provide support for building SCTP as a loadable module.

With this change, a kernel compiled with "options SCTP_SUPPORT" and
without "options SCTP" supports dynamic loading of the SCTP stack.

Currently sctp.ko cannot be unloaded since some prerequisite teardown
logic is not yet implemented. Attempts to unload the module will return
EOPNOTSUPP.

Discussed with: tuexen
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D21997