Use openssh upstream's SSH_AUTH_INFO_0 if present

[opoplawski]

This seems to make things work on Fedora.

[opoplawski]

Hmm, maybe this isn't the best approach as SSH_USER_AUTH has a different (though related) function in upstream openssh. Maybe just a way to define the name of the variable?

[Feandil]

I think that the most forward looking change is to:

• Rewrite the code (function names/comments/...) to point to SSH_AUTH_INFO_0
• Replace the hardcoded environment variable name (SSH_AUTH_INFO_0) with a macro/static variable/..., defaulting to SSH_AUTH_INFO_0. On RHEL7, switch that value to SSH_USER_AUTH.

I guess we should also change the name of that pam module...