This adds the 'mp_reset' keyword to allow sending packets with TCPRST suboption.

Examples:
1.0 R. 1:1(0) ack 1001 win 264 <mp_reset 1>
1.0 R. 1:1(0) ack 1001 win 264 <mp_reset 1 flags [ flag_t ]>

Signed-off-by: Florian Westphal <fw@strlen.de>

mp_reset_multi_v4.pkt - check we can reset a subflow and mptcp
connection is unaffected.

mp_reset_single.pkt - check we can reset single subflow.
Check mp_reset on non-RST packets has no effect.

mp_reset_single_tcp.pkt - check TCP doesn't care about
mp_reset presence.

This also "fixes" existing test cases to expect mp_reset option
in some cases.

Signed-off-by: Florian Westphal <fw@strlen.de>