tcp: improve handling of SYN-ACK segments in TIMEWAIT state

Only consider segments with the SYN bit set and the ACK bit cleared
as "new connection attempts", which result in re-using a connection
being in TIMEWAIT state. This results in consistent handling of
SYN-ACK segments.

Reviewed by: rscheff@
MFC after: 1 week
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D36864