Make system-features a store setting

This seems more correct. It also means one can specify the features a
store should support with --store and remote-store=..., which is useful.
I use this to clean up the build remotes test.

src/libstore/machines.hh

@@ -4,6 +4,8 @@
 
 namespace nix {
 
+class Store;
+
 struct Machine {
 
     const string storeUri;
@@ -28,6 +30,8 @@ struct Machine {
         decltype(supportedFeatures) supportedFeatures,
         decltype(mandatoryFeatures) mandatoryFeatures,
         decltype(sshPublicHostKey) sshPublicHostKey);
+
+    ref<Store> openStore() const;
 };
 
 typedef std::vector<Machine> Machines;