Refactor to remove arg() calls

Chx has called volunteers to help remove calls to arg() in Drupal core, to help prepare for the new approach to URL aliasing.

There are roughly 190 instances in core. For modules, the idea is to move any code which needs to inspect the path to hook_menu(). This may require converting paths to node ids in hook_menu(), grabbing arguments from the path, and passing these off to appropriate handler functions.

Djun