Update (and improve) PAM config(s) for resolving conflict with "phog"

Hi and thanks for maintaining this package!

As you might have noticed, the latest update causes a conflict with phog which used to ship greetd PAM configs as well (my bad, I should've discussed this with you first).

I have pending changes for phog in order to solve this issue, but those also require a Breaks:/Replaces: to be added to greetd to ensure smooth upgrades.

While at it, I updated the PAM config to be nearly identical to those I used for phog, in order to take advantage of the default login config rather than making a custom config out of only common-* ones.