Incorporate changes from openSUSE OBS

* Improve RPM groups
* Improve grammar in RPM subpackage descriptions
* Remove redundant %clean section from rpm spec
* Fix service.d dir which was not referenced by any package
* Use sle_version instead of deprecated leap_version
  see https://en.opensuse.org/openSUSE:Build_Service_cross_distribution_howto#leap_version_macro_.28deprecated.29
* Also add systemd limits file to openSUSE Tumbleweed
* Documentation should not hard-require the program
  as it is technically "usable" without it.