tmpfiles: RFC42-style rules and option to purge targets on change

[bmrips]

Description

This PR accomplishes two things:

• Replace the string-based systemd.user.tmpfiles.rules option by an RFC42-style systemd.user.tmpfiles.settings option.

• Add an systemd.user.tmpfiles.settings.<config-name>.purgeOnChange option to purge the targets of rules on change.

  An example use case of this feature is the programs.glab module: It utilizes tmpfiles to set non-default permissions for its config file. With this PR, the module would be able to reliably remove the config file when it is empty or the module disabled.

The overall structure of the new option (including its defaults) is

systemd.user.tmpfiles.settings."<config-name>" = {
  rules."<path>"."<tmpfiles-type>" = {
    mode = "-";
    user = "-";
    group = "-";
    age = "-";
    argument = "";
  };
  purgeOnChange = false;
};

It resembles NixOS' systemd.tmpfiles.settings option; the only difference is the inserted { rules = ...; purgeOnChange = ...; } submodule to configure purging.

The reason I packed both changes in the same PR is to minimize backwards incompatibilities.

This PR is an advancement of #8084.

Checklist

• Change is backwards compatible.

• Code formatted with nix fmt or
  nix-shell -p treefmt nixfmt deadnix keep-sorted --run treefmt.

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  nix-shell --pure tests -A run.all.

• Test cases updated/added. See example.

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