Restore the ability to have pre-selected category on node/add

As said in #121913: Create Content button in a category?, some users need some sort of "Add content to this category" button. While the button itself is a new feature request (as discussed on that other issue), the destination path for the button is(was) already there: node/add/content_type/###, where ### is the category ID to be pre-selected (default) in the form. There are many ways how to add such a link to a category (some mentioned on the other issue too), including a link inside the body content, hook_link_alter(), hook_nodeapi(), theme, etc. etc. I run it on my site for years, and I believe I'm not the only one.

All these solutions are relying on the path node/add/content_type/### though. It worked just fine on 4.7.x, it worked on 5.x (after a minor bug got fixed), but it's missing on 6.x. I consider this a bug.

Attaching a small patch to put this feature back in, so that these paths work just like they used to be previously - i.e. category ID as additional argument = default category selection.