PAM Module: Add option to find username by token

[strfry]

This is a result of a proof-of-concept, whether libopenkey/PAM can be logged in, just by swiping the RFID card. In this scenario, the username can not be physically entered by the user, but has to be derived from the presented token and the map file. This Pull Request implements this in a crude way, by modifying the logic in check_token_id, to optionally set the username to PAM, instead of checking it. I've yet to find out, how the event of a card swipe can trigger the login process, of a login manager like SLiM or LXDM.