crypto/ssh:improve a new feature to allow user register channel to receive ssh channel close event

[RainyBow]

if userwant to known the channel close event,can use this to register a channel to receive event

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

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[gopherbot]

Message from 李海:

Patch Set 2:

(1 comment)

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[gopherbot]

Message from Ian Lance Taylor:

Patch Set 2:

(1 comment)

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[gopherbot]

Message from Ian Lance Taylor:

Patch Set 2: Hold+1

(1 comment)

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[gopherbot]

This PR (HEAD: 5ff9b59) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/crypto/+/587235.

Important tips:

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• To change your code in response to feedback:
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  • Critical: you must click the blue Reply button near the top to publish your Gerrit responses.
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• The title and description of the GitHub PR are used to construct the final commit message.
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• See the Sending a change via GitHub and Reviews sections of the Contribution Guide as well as the FAQ for details.