bakery_request_account() needs to check for existing email and be better at cleanup

Patch attached.

I believe this is helping cause uid 0 deletions. To reproduce, on a slave site, request a new password by email, where the slave site already has the account. Since bakery does not find an existing account, it goes through most of bakery_request_account(). At the end, it has a duplicate name and the last user_save() fails. The final cleanup does user_delete() on the empty account, deleting uid 0. The code should be more forgiving, but this patch is a start.

http://drupal.org/user/689172 and http://drupal.org/user/21209 existing doesn't help, but that's another issue.

Added:
- Store the $stub_uid; $account->uid is lost during an unsuccessful user_save().
- Clean up the stray stub accounts that have not been used.

I tested a bit locally. The underlying condition on d.o, #1034852: Clean up accounts with case-insensitive duplicate names, is now fixed. The best test might involve hacking user_save() or the master server to sometimes fail.