go.crypto/ssh/terminal: better handling of window resizing.
go.crypto/ssh/terminal: better handling of window resizing.

There doesn't appear to be perfect behaviour for line editing
code in the face of terminal resizing. But this change works
pretty well on xterm and gnome-terminal and certainly a lot
better than it used to.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://golang.org/cl/105580043

golang/term