Enable sctp_sendv/sctp_recvv on Linux

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.

When tearing down a TCP connection, try injecting a RST segment
by pretending to peer has received the last packet as an out of
the blue packet.

Add python also on Linux as it is required by some scripts.

python is required for some test scripts.

Focus on 64-bit systems instead of 32-bit for sctp_primary_addr.pkt on Linux

Before fix:
   sctp_primary_addr.pkt:13: runtime error in setsockopt call: Expected result 0 but got -1 with errno 22 (Invalid argument)

After fix:
   test passed both on Linux x86_64 and ppc64le

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

Fix the control message length issue on Linux