Additional username validation on user creation and edit necessary

Users should not be able to choose a username already used as e-mail address of another user nor an e-mail address already used as somebody else username.

Example:

1. User A registers with username "drupal" and e-mail "drupal@example.com"
2. User B registers with username "drupal@example.com" and e-mail "example@example.com"

• User A won't be able to login with it's e-mail address as user B will be loaded in login_emailusername_user_login_validate()
• If user A tries to often to login, user B might be blocked