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
Using pkgs.lib on the spine of module evaluation is problematic
because the pkgs argument depends on the result of module
evaluation. To prevent an infinite recursion, pkgs and some of the
modules are evaluated twice, which is inefficient. Using ‘with lib’
prevents this problem.
