linger-users: fix shellcheck warnings

Running with systemd.enableStrictShellChecks with lingering users causes
failures due to parsing the output from `ls`.  Rewrite the script to
avoid parsing ls, and instead rely on loginctl enable-linger and
disable-linger commands being idempotent and run them unconditionally.

This also fixes a bug where the systemd unit for adding and removing
lingering user configuration is only enabled if there are users
configured with lingering in the NixOS configuration.  This means that
if a NixOS system is built with some lingering users, then the linger
configuration is removed from all those users, the script to disable
lingering won't be run, and those users will incorrectly continue to
have lingering enabled.

Fixes #418101.

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