Use drupal_override_function's hook_function_overrides for runkit

I spent some time setting up this module with runkit last night, wrote a blog post about it, and think it's very cool. One suggestion I would make, though, is to implement runkit through the Drupal Override Function module, which provides a hook_function_overrides for either runkit or APD. If path_alias_xt implemented the hook, you could simply write in the docs, "enable drupal_override_function for this additional functionality, see their docs for instructions." Then each module handles its "core competency," and if more and more modules start using runkit, they can all stick with a standard method/API.
The attached patch modifies the code to handle this. (It does not yet modify the documentation to explain the change.)
Note that on my setup at least, this also required #961814: Error "canary mismatch on efree" from runkit_function_rename to work, because of an apparent bug in PHP or runkit.