exp/ssh: ensure initial window advertisement is not lost
exp/ssh: ensure initial window advertisement is not lost

Some remote servers send a 0 window size in the channel
open confirm msg, others send a non zero window size. Make
sure this initial advertisement is not lost.

R=agl, rsc, cw
CC=golang-dev
http://codereview.appspot.com/5372083

Committer: Adam Langley <db68fba232b2c49ed3e9d3081df349dde7b61dfd@golang.org>

gnanderson/go,tv42/old-go,apparentlymart/golang,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,dsriram/golang,apparentlymart/golang,tv42/old-go,gnanderson/go,dsriram/golang,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,dsriram/golang,gnanderson/go,dsriram/golang,dsriram/golang,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,dsriram/golang,gnanderson/go,pombredanne/go-deleteme,dsriram/golang,gnanderson/go,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,gnanderson/go