<!-- Add the related story/sub-task/bug number, like Resolves#123, or
remove if NA -->
**Related issue:** Resolves#33381 unreleased Linux unlock bug.
# Checklist for submitter
## Testing
- [x] QA'd all new/changed functionality manually
Fixes#31291
# Checklist for submitter
- [x] Changes file added for user-visible changes in `changes/`,
`orbit/changes/` or `ee/fleetd-chrome/changes`.
See [Changes
files](https://github.com/fleetdm/fleet/blob/main/docs/Contributing/guides/committing-changes.md#changes-files)
for more information.
## Testing
- [x] QA'd all new/changed functionality manually
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
- New Features
- Linux lock now switches Ubuntu + GDM systems to text mode to avoid GUI
issues.
- Persistent lock message is shown and survives reboots.
- Unlock restores the original GUI mode automatically when applicable.
- Bug Fixes
- Prevents black-screen behavior on Ubuntu + GDM after locking by
rebooting to text mode.
- Ensures lock message consistently appears across sessions.
- Improves reliability of session handling during lock/unlock.
- Chores
- Added change note describing the updated Linux lock behavior.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
#20370
Part 2 to #20699. Apparently `systemd` now automatically deletes
`/etc/nologin` on startup. In the previous PR, rebooting the machine
would remove the nologin file and allow users to login. This PR masks
the service that performs the deletion, preventing it from running.
The message displayed to the user will be what is specified in [this
file](7767896d12/tmpfiles.d/systemd-nologin.conf (L10)).
It's not the best, but I suspect messing with too many systemd files
could come back to bite us in the future if things change, so I'll leave
it as-is.
