server.go/channel.go:
* rename Server to ServerConfig to match Client.
* rename ServerConnection to ServeConn to match Client.
* add Listen/Listener.
* ServerConn.Handshake(), general cleanups.

client.go:
* fix bug where fmt.Error was not assigned to err

R=rsc, agl
CC=golang-dev
https://golang.org/cl/5265049