varnish: fix localstatedir for varnish* tools (#17508)

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