Update client.go

[Moorong]

Client.NewSession方法仅仅返回了Session，而打开的channel通道却没有用武之地，且使用方无法获取到该通道，因为是私有属性，如果想要使用channel，需要再次调用openChanne()方法。本人进行服务器和交换机ssh远程登录时，使用到了该源码，服务器可以正常使用，但交换机只能一个Session一个Channel。本人添加NewSessionAndChannel()方法将Session中的channel及request一并返回，使用该channel与交换机及服务器都能正常通信。

[gopherbot]

This PR (HEAD: ae31a8bd46db90f4b0a6607db526dd28a13995ce) has been imported to Gerrit for code review.

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[gopherbot]

Message from Ian Lance Taylor:

Patch Set 1: Hold+1

(1 comment)

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[Moorong]

The Client.NewSession method only returns the Session, but the opened channel is useless, and the user cannot obtain the channel because it is a private property. If you want to use the channel, you need to call the openChanne() method again. When I log in to the server and the switch ssh remotely, I use this source code, the server can be used normally, but the switch can only have one Session and one Channel, and cannot open another channel again. I add the NewSessionAndChannel() method to return the channel and request in the Session together, and use the channel to communicate with the switch and the server normally.