Install files to `/usr/lib/pam.d` as well

As said in #1 I'd like to help with the migration. For now I decided that additionally installing the files to their /usr/ equivalent is the "best" solution, since that preserves the workflow of people comparing .pacnew files to update their config, while allowing the complete removal of the files to reset to defaults.

See: archlinux/ideas#1

Partially fixes: #1