Fix invocation of hook_signup_cancel() so that the record in {signup_log} is still there

Over at #325237-2: Allow users (even anonymous) to cancel signup via secure URL, Sid_M raises the point about saving canceled signups into another table. I could use that functionality myself. So, either signup should do this itself, or at least it should provide a hook for other modules to do so.

Maybe it's just a question of changing the order of operations in signup_cancel_signup(). Currently, we delete the record first, then invoke hook_signup_cancel(). Arguably, we should invoke the hook first to give other modules a chance to do whatever they want with the {signup_log} record, and then actually remove it. I'd like more input on this before adding Yet another hook (hook_signup_pre_cancel()?).