A lot of two-factor modules, such as `pam_oath` and `pam_u2f`, require adding custom preludes to `/etc/pam.d/system-auth` in order to function. While it makes sense to not modify the pam configuration by default when installing these packages, it would be beneficial to add a default prelude to this file of the form `auth include system-auth-custom`, with an empty stub at `/etc/pam.d/system-auth`, so that users can add their prelude there without having to modify this file.

I feel that most users would prefer to add extra lines to the pam auth, instead of removing them, so, it would be helpful to allow them to do so without having to modify the file directly. That way, if additional lines are added on update, the users don't need to merge changes with pacdiff.

Obviously, this still has the potential to break setups if users aren't careful with their preludes, but I feel that this is part of the standard arch update process anyway.