Add re-setcred feature

Hello !

Thank youy for your rewrite of xtrlock in python with libpam support ! I like xtrlock but it was improper to use with network-based (or just modern) authentication solutions.
I found your port lacked the ability to reset my credentials at login though, so I don't have to retype 'kinit' every once in a while to get a new Kerberos ticket (yeah, I'm lazy). So here is the patch to do just that. :]

Regards.