nix verify: Support checking against signatures in other stores

Typical usage is to check local paths using the signatures from a
binary cache:

  $ nix verify-paths -r /run/current-system -s https://cache.nixos.org
  path ‘/nix/store/c1k4zqfb74wba5sn4yflb044gvap0x6k-nixos-system-mandark-16.03.git.fc2d7a5M’ is untrusted
  ...
  checked 844 paths, 119 untrusted