Pull up following revision(s) (requested by kardel in ticket #1602):

sys/netinet/tcp_input.c: revision 1.419

Fix fast path for uni directional transfers

pure ACK case:
drag snd_wl2 along so only newer
ACKs can update the window size.

also avoids the state where snd_wl2
is eventually larger than th_ack and thus
blocking the window update mechanism and
the connection gets stuck for a loooong
time in the zero sized send window state.

see PR/kern 55567

ok thorpej@, also found in FreeBSD

(martin)