go.crypto/ssh: assorted close related fixes

Fixes golang/go#3810.

Fixes chanWriter Write after close behaviour bug.

Fixes serverChan writePacket after close bug.

Addresses final comments by agl on 6405064, plus various cleanups.

R=agl, kardianos, gustav.paul, fullung
CC=golang-dev
https://golang.org/cl/6479056

ssh/server.go

@@ -536,7 +536,7 @@ func (s *ServerConn) Accept() (Channel, error) {
 
 			// TODO(dfc) s.lock protects s.channels but isn't being held here.
 			for _, c := range s.channels {
-				c.dead = true
+				c.setDead()
 				c.handleData(nil)
 			}