m_ldapauth: fix providing username in PASS

m_ldapauth: fix providing username in PASS

This has never worked because it never prepended attribute= to the
search string. It also would prefer matching nick/ident over the user
string provided which is inconsistent, since your ability to
authenticate using PASS would depend on a ldap entry not existing with
your current nick/ident.

Instead, use username provided in PASS and then fallback to nick/ident.