net: sctp: introduce uapi header for sctp

This patch introduces an UAPI header for the SCTP protocol,
so that we can facilitate the maintenance and development of
user land applications or libraries, in particular in terms
of header synchronization.

To not break compatibility, some fragments from lksctp-tools'
netinet/sctp.h have been carefully included, while taking care
that neither kernel nor user land breaks, so both compile fine
with this change (for lksctp-tools I tested with the old
netinet/sctp.h header and with a newly adapted one that includes
the uapi sctp header). lksctp-tools smoke test run through
successfully as well in both cases.

Suggested-by: Neil Horman <nhorman@tuxdriver.com>
Cc: Neil Horman <nhorman@tuxdriver.com>
Cc: Vlad Yasevich <vyasevich@gmail.com>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>