Use {signup_log}.sid to simplify node/N/signup code and fix the API for hook_signup_cancel()

This removes a bunch of ugly code. The functionality is the same, yet the patch removes 57 lines and only adds 41 (9 of which are expanded PHPdoc comments). ;) I think the API for signup_cancel_signup() is more sane, now, too, given the sid key. And, since it's the 6.x-2.* series, I don't mind breaking compatibility to fix the API.

Any thoughts/objections/reviews/testing before I commit?

Great, thanks for the review. Committed to HEAD: http://drupal.org/cvs?commit=159262

Actually, the more I think about it, I think the new API for hook_signup_cancel() should be:

/**
 * @param $node
 *   The fully-loaded node object that the signup is being canceled from.
 * @param $signup
 *   An object containing all the known information about the signup being canceled.
 *   Contains all the data from the {signup_log} row representing the canceled signup.
 *   See the schema definition for descriptions of each field and what they represent.
 */
function hook_signup_cancel($node, $signup) {
  ...
}

We've already got the fully loaded $signup object in signup_cancel_signup(). Why just pass the sid and uid, when we can pass an object with everything? Seems much more useful that way, doesn't cost us anything, and if we're already breaking the API, we might as well do it right. ;)

Thoughts?

FYI: After further consideration, I just backported this (and related changes) into DRUPAL-6--1: http://drupal.org/cvs?commit=159750

Furthermore, as I think about hook_signup_cancel, it seems like the API should really be: hook_signup_cancel($signup, $node). The $signup object is more important than the $node, so it should come first. And, since we're breaking the API, we might as well break it all the way. Attached patch does this, and adds a signup.api.php file to document signup hooks (along the lines of the new convention in D7 core).

Great, we're all agreed. Committed to HEAD and DRUPAL-6--1. Yay. ;)