tcp: Handle <RST,ACK> in SYN-RCVD

Patch base stack to correctly handle the RST bit independently
of other header flags per TCP RFC.

MFC after: 1 week
Reviewed By: tuexen, transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D40982

(cherry picked from commit b352ef58c29b1f2967e358c147e0f7faf8cfbef3)