Add systemd snippets and use macros for paths
    
    Add the required systemd snippets[1] and use macros for the udev
    rules and unit directories.
    Also mark "%{_datadir}/gtk-doc" as directory as to avoid warnings
    about duplicated entries, e.g:
      "warning: File listed twice: /usr/share/gtk-doc/html/UPower"
    
    [1] https://docs.fedoraproject.org/en-US/packaging-guidelines/Scriptlets/#Systemd