io.files: use initialize to set resource-path

If a resource path is already set in the image, don't change it. Should make #364 work a bit more as expected, and let us set the resource path for deployed Mac OS X bundles.

core/io/files/files.factor

@@ -65,5 +65,5 @@ PRIVATE>
     cwd current-directory set-global
     OBJ-IMAGE special-object alien>native-string cwd prepend-path \ image set-global
     OBJ-EXECUTABLE special-object alien>native-string cwd prepend-path \ vm set-global
-    image parent-directory "resource-path" set-global
+    "resource-path" [ image parent-directory ] initialize
 ] "io.files" add-startup-hook