Allow logging in by username OR e-mail

This patch adds the option that when activated, will allow users to login using either their username OR e-mail address.

I use it with Logintoboggan and it seems to be working fine.

The patch needs serious review as it removes the $server bit from ldapauth_auth() because it didn't seem to be used anywhere through the code (and obviously, causes problems with e-mails because the username will look like this at the end: foobar@example.com@example.com).