webui: fix possible issues due to PAM misconfiguration

[fbergkemper]

Check if UsePAMAuthentication is enabled on configured console
in DIR, if not do not proceed with authentication.

Fixes #1191: The web interface runs under any login and password

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[joergsteffens]

I added an error message for a specific case and tried to clarify the example configuration. Besides of this, I fully agree with this change.
Feel free to stash my commit into yours.

[fbergkemper]

Thanks for the additions and the review.