[Dovecot] plain auth problem with beta4

[Mark Davies]

Using kmail with PLAIN authentication worked fine with beta3 but trying it 
with beta4 authentication fails.

This is because kmail sends  "username \0 username \0 password" in the 
authorization token and the new code to call 
auth_request_set_login_username() when supplied an authid must be returning 
failure (certainly commenting this code out returns to the beta3 behaviour of 
successfully authenticating).  I had a quick look at the routine to see why 
it might be failing but didn't really work out what its supposed to do.

cheers
mark