exp/terminal: rename shell to terminal and add SetSize

exp/terminal: rename shell to terminal and add SetSize

It was never really a shell, but the name carried
over from SSH's ServerShell.

Two small functional changes:

Add SetSize, which allows the size of the terminal
to be changed in response, say, to an SSH message.

Don't write the prompt if there's already something
on the current line.

R=rsc
CC=golang-dev
https://golang.org/cl/5376066