exp/ssh: Have Wait() return an *ExitError

exp/ssh: Have Wait() return an *ExitError

I added the clientChan's msg channel to the list of channels that are closed in mainloop when the server sends a channelCloseMsg.

I added an ExitError type that wraps a Waitmsg similar to that of os/exec. I fill ExitStatus with the data returned in the 'exit-status' channel message and Msg with the data returned in the 'exit-signal' channel message.

Instead of having Wait() return on the first 'exit-status'/'exit-signal' I have it return an ExitError containing the status and signal when the clientChan's msg channel is closed.

I added two tests cases to session_test.go that test for exit status 0 (in which case Wait() returns nil) and exit status 1 (in which case Wait() returns an ExitError with ExitStatus 1)

R=dave, agl, rsc, golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5452051