Mitigate a few false positives in "functions that implement this hook"

Migrated issue

Reported by: webchick

Glancing through http://api.drupal.org/api/drupal/modules%21system%21system.api.php/function/implementations/hook_menu/7 seems like we might be able to eliminate a few of those with some patterns like:

1) If the function starts with an underscore (e.g. http://api.drupal.org/api/drupal/modules%21book%21book.module/function/_book_flatten_menu/7), it is not a hook implementation.

2) If the function is part of a class definition (e.g. http://api.drupal.org/api/drupal/modules%21user%21user.test/function/UserAccountLinksUnitTests%3A%3AtestSecondaryMenu/7), it is not a hook implementation.

3) If the part of the function name before _menu doesn't exactly match a module name (e.g. http://api.drupal.org/api/drupal/modules%21book%21book.module/function/book_get_flat_menu/7), it is not a hook implementation.

Just some ideas to make this already kick-ass functionality even better. :)