maybe this should include the path to the Nix store and a sanity check
should be added to make sure that if we check the file in $STATEDIR that
it is indeed from an instance of the service that we want to start. This
adds complexity, because how should we deal with the situation of they are
not equal.
TODO: add a "status" command, or a "version" command, which displays which
service is currently registered with the system as "running"
svn path=/nixpkgs/trunk/; revision=5015
