x/crypto/ssh: add missing subsystem handling

[skinowski]

Subsystem requests are aliases that are similar to exec or
shell requests. The server can respond to subsystem requests
with exit status or exit signal. This means in session handling
code, subsystem requests also need be started and waited.

Also see RFC4254:
https://datatracker.ietf.org/doc/html/rfc4254#section-6.5

[gopherbot]

This PR (HEAD: 5608a4f) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/crypto/+/343749 to see it.

Tip: You can toggle comments from me using the comments slash command (e.g. /comments off)
See the Wiki page for more info

[skinowski]

Fixes golang/go#35025

[gopherbot]

Message from Go Bot:

Patch Set 1:

Congratulations on opening your first change. Thank you for your contribution!

Next steps:
A maintainer will review your change and provide feedback. See
https://golang.org/doc/contribute.html#review for more info and tips to get your
patch through code review.

Most changes in the Go project go through a few rounds of revision. This can be
surprising to people new to the project. The careful, iterative review process
is our way of helping mentor contributors and ensuring that their contributions
have a lasting impact.

During May-July and Nov-Jan the Go project is in a code freeze, during which
little code gets reviewed or merged. If a reviewer responds with a comment like
R=go1.11 or adds a tag like "wait-release", it means that this CL will be
reviewed as part of the next development cycle. See https://golang.org/s/release
for more details.

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/343749.
After addressing review feedback, remember to publish your drafts!

[kobrineli]

@hanwen
Hi! Can you review this PR, please?

[hanwen]

I have left Google, so I am not paid for work on Go anymore. You can try @FiloSottile . I should mention that the pending change still has no test.