lksctp-tools has supported sctp_sendv and sctp_recvv.

Signed-off-by: Jianwen Ji <jijianwen@gmail.com>

The assert() calls clarify and ensure which thread the functions
is allowed to run in.

Ensure that
 0.00 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+1.00 close(3) = -1
is handled correctly.

Still missing: An implementation of is_thread_sleeping() for MacOS.

When using TFO and the SYN carries user data, the sending of
the SYN-ACK is delayed and accept returns. This allows the
application to provide user data to be put in the SYN-ACK.

Detect a runtime error if the bytes argument is not as expected.