Systemd Support

[sternenseemann]

Add systemd support for the server executable (socket activation and notify service type).

Work in Progress, see the systemd-support branch

Current Progress:

• [x] Write rudimentary Systemd-Support using a patched version of the systemd haskell package
• [x] Further testing
• [x] Either rewrite the integration or patch the systemd haskell package, so spacecookie does not have to depend on network
  • [x] Wait for feedback on the PR
• [x] Update documentation and suggested systemd .service files.
• [x] Test on non-systemd and non-linux (see #9) systems
• [ ] Get a NixOS service into nixpkgs

This is the new feature I am planning for 0.2.1.0.