Move away from nss-altfiles
### Description
We have hit a lot of issues with how we split users between the configuration in the image (`/usr/lib/passwd` & `group`, `/usr/etc/passwd`, `group`, `shadow` & `gshadow`) and the local configuration (`/etc/passwd`, `group`, `shadow` & `gshadow`) using `nss-altfiles`:
- https://forge.fedoraproject.org/atomic-desktops/tracker/issues/90
- https://forge.fedoraproject.org/kde/tracker/issues/684
- https://github.com/fedora-silverblue/issue-tracker/issues/362
- https://docs.fedoraproject.org/en-US/atomic-desktops/troubleshooting/#_unable_to_add_user_to_group
- https://forge.fedoraproject.org/kde/tracker/issues/109#issuecomment-555155
and others in Fedora bootc & Fedora CoreOS.
The solution is mostly described by https://github.com/coreos/fedora-coreos-tracker/issues/1599: We should stop using `nss-altfiles`.
There are two main tasks to make that happen:
- Build images without `nss-altfiles` that work properly:
- They should likely ship with a default copy of `/usr/etc/passwd`, `group`, `shadow` & `gshadow`, maybe only with a limited set of static users (the ones used by files & packages in the image)
- And then use `systemd-sysusers` for the rest
- Create a migration path for existing systems to merge the config from the image into the local config, preserving local modifications.
- This is probably something that would be good to put in bootc or rpm-ostree.
### Additional information
See also:
- https://gitlab.com/fedora/bootc/tracker/-/work_items/50
- https://gitlab.com/fedora/bootc/tracker/-/work_items/31
- https://github.com/bootc-dev/bootc/issues/673
- https://github.com/bootc-dev/bootc/issues/1179
- https://github.com/bootc-dev/bootc/issues/1263
- https://github.com/coreos/rpm-ostree/issues/49
- https://github.com/coreos/fedora-coreos-tracker/issues/1599
- https://github.com/coreos/fedora-coreos-tracker/issues/155
- https://forge.fedoraproject.org/atomic-desktops/tracker/issues/108
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