Pass ‘config’ to stdenv/generic/default.nix

This is a backport of cf8daf63120adedbeaf5bc8c2f396be2496a741e in
stdenv-updates (sans the userHook stuff).
Add config option ‘allowUnfree’

If set to false, mkDerivation will throw an exception if a package has
an unfree license.  ‘release-lib.nix’ uses this to enforce that we
don't build unfree packages as part of the Nixpkgs channel.  Since
this is set through Nixpkgs' ‘config’ argument, it's more finegrained
than $HYDRA_DISALLOW_UNFREE.