LDAP: Fall back to functional level of Windows Server 2003
The newest functional level we branch for is currently
DS_BEHAVIOR_WIN2003. Therefore (and also because extended support for
Windows server 2003 ends in 2015) we can safely set the functional level
to 2003 if the attribute is present but not a known value.
Reviewed-by: Pavel Březina <pbrezina@redhat.com>