This CL replaces the Cmd type with a Session type representing
interactive channels. This lays the foundation for supporting
other kinds of channels like direct-tcpip or x11.

client.go:
* replace chanlist map with slice.
* generalize stdout and stderr into a single type.
* unexport ClientChan to clientChan.

doc.go:
* update ServerConfig/ServerConn documentation.
* update Client example for Session.

message.go:
* make channelExtendedData more like channelData.

session.go:
* added Session which replaces Cmd.

R=agl, rsc, n13m3y3r, gustavo
CC=golang-dev
https://golang.org/cl/5302054