Allow /usr/etc/ in abstractions/authentication
openSUSE (and hopefully some other distributions) work on moving shipped config files from /etc/ to /usr/etc/ so that /etc/ only contains files written by the admin of each system.
See https://en.opensuse.org/openSUSE:Packaging_UsrEtc for details and the first moved files.
Updating abstractions/authentication is the first step, and also fixes bugzilla.opensuse.org/show_bug.cgi?id=1153162
I propose this patch for 2.12..master - distros using older versions won't move their config files around ;-)