go.crypto/ssh: add Stderr() in Channel interface.
go.crypto/ssh: add Stderr() in Channel interface.

Adds support for piping Stderr to the client.

R=golang-dev, dave, agl
CC=golang-dev
https://golang.org/cl/5674081

wooutl/golangcrypto,cmars/tools,ScriptRock/crypto,kim-racktop/crypto,golang/crypto,tca/crypto,conseweb/golangcrypto,cheffo/crypto,Carrotman42/crypto,progrium/crypto,juniorz/crypto,Carrotman42/crypto,cmars/tools,golang/crypto,lucevers/crypto,guaiguaihw/crypto,a-palchikov/crypto,tca/crypto,golang/crypto,lucevers/crypto,guaiguaihw/crypto,a-palchikov/crypto,ScriptRock/crypto,CodyGuo/crypto,juniorz/crypto,conseweb/golangcrypto,thomaso-mirodin/crypto,thomaso-mirodin/crypto,cmars/tools,kim-racktop/crypto,zj8487/crypto,progrium/crypto,cmars/tools,mattharden/crypto,pki-io/crypto,bford/golang-x-crypto,zj8487/crypto,CodyGuo/crypto,wooutl/golangcrypto,bford/golang-x-crypto,cheffo/crypto,pki-io/crypto,mattharden/crypto