Path aliases - controlling outgoing alias

Here's a patch to implement 2/3 of my previous suggestions.

1. A status column is added to the url_alias table. A 0 (zero) value indicates an alias explicitly created by a user or administrator; a 1 (one) value indicates an alias created by an automated process. No core code support is required here - aliases created by core methods (the node form or url aliases settings) will default to 0. The pathauto module (or any other module that wishes to automatically generate aliases) will create them with a status of 1, and when updating aliases will only delete/modify aliases with a status of 1, so that aliases created explicitly by users are not blown away.

2. A weight column is added to the url_alias table. As path aliasing exists now, multiple aliases may be created for a given path but there is no control over which one is generated in the outgoing direction (as in listings). This patch adds the support for editing weights for aliases to the url aliases settings (and both status and weight become optional arguments to path_set_alias()), and drupal_lookup_path() is modified to pick the lowest weighted alias in response to an 'alias' call (if there's a tie for lowest weight, it's the same result as before, which is most likely lowest pid === first created).

I really wanted to implement a method for having aliases that produce a 301 (Move Permanently) response, particularly if you want to change the alias for a page without abandoning any bookmarks/search engine links that are already out there - but I started on it and realized it will require more thought to integrate cleanly... It might just be easier to do that using conf_url_rewrite()...

Thanks for your consideration...