x/crypto/ssh: send ext-info-c only once

[peterverraedt]

In accordance to RFC8308, send ext-info-c only during the first key
exchange. Some server implementations such as OpenSSH 7 will send an
extInfoMsg message each time when ext-info-c is received. This results
in a closed connection, as our client does not expect this message while
handling the mux.

See https://bugzilla.mindrot.org/show_bug.cgi?id=2929 regarding the
behaviour of OpenSSH if it sees ext-info-c in later key exchanges.

Fixes golang/go#51808

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Patch Set 2: Run-TryBot+1 Code-Review+2

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Patch Set 5:

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Patch Set 5: Code-Review+2

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Patch Set 5: Run-TryBot+1

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Patch Set 5: Trust+1

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This PR is being closed because golang.org/cl/394134 has been merged.