The linger-users systemd unit runs a Bash script. To allow this to be
avoided for the bashless profile, provide an option to have NixOS not
manage lingering for any users.
To make this feasible, add the possibility for each individual user
account to not have its lingering configuration managed by NixOS at all,
and make this the default from 26.05. In practice, this won't result in
a change of behaviour except for people who manually use `loginctl
enable-linger` commands to add lingering for some user accounts, then
rely on NixOS to disable lingering the next time the systemd units are
restarted.
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