The varnish tools (varnishstat, varnishlog, ...) tried to load the VSM
file from a spurious var directory in the Nix store. Fix the default so
the tools "just work" when also keeping services.varnish.stateDir at the
default.
Notes:
- The tools use $localstatedir/$HOSTNAME so I've adapted the default for
stateDir as well to contain hostName.
- Added postStop action to remove the localstatedir. There is no point
in keeping it around when varnish does not run, as it regenerates it
on startup anyway.
Fixes#7495
ea7e705cd9