Split OptionalPathSetting from PathSetting

Rather than doing `allowEmpty` as boolean, have separate types and use
`std::optional`. This makes it harder to forget the possibility of an
empty path.

The `build-hook` setting was categorized as a `PathSetting`, but
actually it was split into arguments. No good! Now, it is
`Setting<Strings>` which actually reflects what it means and how it is
used.

Because of the subtyping, we now also have support for
`Setting<std::optional<String>>` in general. I imagine this can be used
to clean up many more settings also.

src/libstore/globals.cc

@@ -100,7 +100,10 @@ Settings::Settings()
     if (!pathExists(nixExePath)) {
         nixExePath = getSelfExe().value_or("nix");
     }
-    buildHook = nixExePath + " __build-remote";
+    buildHook = {
+        nixExePath,
+        "__build-remote",
+    };
 }
 
 void loadConfFile()