go.crypto/ssh: never send more data than maxpacket

RFC 4254 s5.2 is clear that a client must never send a data
packet larger than the value of maximum packet supplied by the
remote side during channel setup. The client was not honoring
this value, in fact it wasn't even recording it.

Thanks to Albert Strasheim for the bug report.

R=agl, fullung
CC=golang-dev
https://golang.org/cl/6448128

ssh/server.go

@@ -569,14 +569,15 @@ func (s *ServerConn) Accept() (Channel, error) {
 						conn:      s,
 						remoteId:  msg.PeersId,
 						remoteWin: window{Cond: newCond()},
+						// TODO(dfc) assert this param is < 2^31.
+						maxPacketSize: msg.MaxPacketSize,
 					},
-					chanType:      msg.ChanType,
-					maxPacketSize: msg.MaxPacketSize,
-					extraData:     msg.TypeSpecificData,
-					myWindow:      defaultWindowSize,
-					serverConn:    s,
-					cond:          newCond(),
-					pendingData:   make([]byte, defaultWindowSize),
+					chanType:    msg.ChanType,
+					extraData:   msg.TypeSpecificData,
+					myWindow:    defaultWindowSize,
+					serverConn:  s,
+					cond:        newCond(),
+					pendingData: make([]byte, defaultWindowSize),
 				}
 				c.remoteWin.add(msg.PeersWindow)
 				s.lock.Lock()