signed-binary-caches -> require-sigs

Unlike signed-binary-caches (which could only be '*' or ''),
require-sigs is a proper Boolean option. The default is true.

src/libstore/globals.hh

@@ -281,6 +281,11 @@ public:
     Setting<std::string> signedBinaryCaches{this, "*", "signed-binary-caches",
         "Obsolete."};
 
+    Setting<bool> requireSigs{this, signedBinaryCaches == "*", "require-sigs",
+        "Whether to check that any non-content-addressed path added to the "
+        "Nix store has a valid signature (that is, one signed using a key "
+        "listed in 'trusted-public-keys'."};
+
     Setting<Strings> substituters{this,
         nixStore == "/nix/store" ? Strings{"https://cache.nixos.org/"} : Strings(),
         "substituters",

src/libstore/local-store.hh

@@ -97,7 +97,7 @@ public:
 private:
 
     Setting<bool> requireSigs{(Store*) this,
-        settings.signedBinaryCaches != "", // FIXME
+        settings.requireSigs,
         "require-sigs", "whether store paths should have a trusted signature on import"};
 
     PublicKeys publicKeys;