exp/ssh: general cleanups for client support
common.go:
* simplify findAgreedAlgorithms.
* add channelExtendedData support.
messages.go:
* add clientExtendedData.
server.go:
* use simplified findAgreedAlgorithms.
server_shell.go:
* fix shadowed err return value.
transport.go:
* introduce separate cipher, mac and compression for each direction.
* added filteredConn and packetWriter interfaces.
* newTransport requires a source of randomness.
R=golang-dev, agl, rsc
CC=golang-dev
https://golang.org/cl/
5285044