Add a Config class to simplify adding configuration settings

The typical use is to inherit Config and add Setting<T> members:

  class MyClass : private Config
  {
    Setting<int> foo{this, 123, "foo", "the number of foos to use"};
    Setting<std::string> bar{this, "blabla", "bar", "the name of the bar"};

    MyClass() : Config(readConfigFile("/etc/my-app.conf"))
    {
      std::cout << foo << "\n"; // will print 123 unless overriden
    }
  };

Currently, this is used by Store and its subclasses for store
parameters. You now get a warning if you specify a non-existant store
parameter in a store URI.

src/libstore/local-store.hh

@@ -67,6 +67,8 @@ private:
 
 public:
 
+    PathSetting realStoreDir_;
+
     const Path realStoreDir;
     const Path dbDir;
     const Path linksDir;
@@ -76,7 +78,9 @@ public:
 
 private:
 
-    bool requireSigs;
+    Setting<bool> requireSigs{(Store*) this,
+        trim(settings.get("signed-binary-caches", std::string("*"))) != "",
+        "require-sigs", "whether store paths should have a trusted signature on import"};
 
     PublicKeys publicKeys;