crypto/ssh: Handle msgUserAuthBanner during keyboard-interactive auth.
crypto/ssh: Handle msgUserAuthBanner during keyboard-interactive auth.

R=agl, golang-dev
CC=golang-dev
https://golang.org/cl/12983046

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