Correctly set up the initial TCP congestion window in all cases,

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