#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.
