go.crypto/ssh: Increase window size.

Increase window size for channels (session and tcpip) to 64 *
max packet size (32 KB), which is the same value that OpenSSH
uses. Also breaks out the relevant harcoded constants into named
constants in channel.go.

Fixes golang/go#6675.

R=golang-dev, dave, hanwen, agl
CC=golang-dev
https://golang.org/cl/18120043

ssh/session.go

@@ -567,8 +567,8 @@ func (c *ClientConn) NewSession() (*Session, error) {
 	if err := c.transport.writePacket(marshal(msgChannelOpen, channelOpenMsg{
 		ChanType:      "session",
 		PeersId:       ch.localId,
-		PeersWindow:   1 << 14,
-		MaxPacketSize: 1 << 15, // RFC 4253 6.1
+		PeersWindow:   channelWindowSize,
+		MaxPacketSize: channelMaxPacketSize,
 	})); err != nil {
 		c.chanList.remove(ch.localId)
 		return nil, err