MFC r360477: Correctly set up the initial TCP congestion window in all cases

by not including the SYN bit sequence space in cwnd related calculations.
Snd_und is adjusted explicitly in all cases, outside the cwnd update, instead.

This fixes an off-by-one conformance issue with regular TCP sessions
not using Appropriate Byte Counting (RFC3465), sending one more
packet during the initial window than expected.

PR: 235256
Reviewed by: tuexen (mentor), rgrimes (mentor, blanket)
Approved by: tuexen (mentor), rgrimes (mentor, blanket)
MFC after: 3 weeks
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D19000