xserver: sddmv2 patchset: sync with upstream

[WavyEbuilder]

Making an issue for this just to note I'm working on it and to put any design questions along the way as well as to note the existence of the problem.

A while back in commit a9fe3da, the changes with upstream for sddm have been reverted, which isn't exactly ideal (especially as the xdm user has a valid use case with display mangers having a sort of hybrid between normal users and system users as they make use of seats, etc).

The reasoning for the revert is given in commit dada9b3 as follows:

This commit added the sddm user to the xserver module.
This caused problems loading the xserver module if the user did not
exist on the system.

However, this doesn't appear to be a Gentoo specific issue. Upstream hit this issue in SELinuxProject/refpolicy#488 and resolved it in SELinuxProject/refpolicy@b6998e4, allowing us to now look at getting back into sync with upstream for the xserver policy module.