Due to this (and previous) requests, I've simply added back the 
'userdn' hook for Passwd 5.0.2.

We use LDAP with DNs like this:
uid=<user>, ... , dc=<domain>
but we authenticate users with their e-mail address <user>@<domain>

We used the usedn function in hooks.php to split the e-mail address 
and generate correct DN for binding to LDAP server.

But now userdn hook is gone. In backends.local.php we can use userdn 
but there is no way to split the userid in a way we can use it. The 
only substitution that happens is for %u.

I though it will be useful if we can use parts of the userid. Proposed 
patch splits userid into two <user> and <domain> and replaces %l with 
<user> and %d with <domain> in userdn attribute.

Cheers,
Stoyan