ssh: added RequestSubsystem().
ssh: added RequestSubsystem().

RequestSubsystem requests the association of a subsystem with the ssh
session on the remote host. A subsystem is a predefined command that
runs in the background when the ssh session is initiated (i.e. sftp).

R=golang-dev, agl
CC=golang-dev, gustav.paul
https://golang.org/cl/6295096

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