Permission to search by e-mail

Firstly, there's a 'bug' with the current perms -- if you disable searching by e-mail, folks who know the URL scheme can still search using /finduser/email/blah@example.com. Secondly, it would be nice to allow a fine-tuning of permissions. If for no other reason than the default page callback uses user_access('access user profiles'), which means it's an all-or-nothing for profiles and searching by e-mail.

This patch solves both issues by introducing a new permission, 'find user search e-mail'. It also fixes the first bug by ensuring the page callback for e-mail searches honors both the main setting and the user access permission.