ssh/server: handle repeated SSH authentication service requests

[9init]

Fixes golang/go#75268

This change improves the SSH server authentication logic by allowing
repeated authentication service requests (SSH_MSG_SERVICE_REQUEST)
during the authentication phase.

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Patch Set 1:

(1 comment)

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Message from Nicola Murino:

Patch Set 2:

(1 comment)

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Message from t hepudds:

Patch Set 2:

(1 comment)

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Message from Ahmed Hesham:

Patch Set 3:

(2 comments)

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