WL#539: Create automatically KEY for AUTO_INCREMENT

Affects: Server-5.0   —   Status: Un-Assigned   —   Priority: Medium

If AUTO_INCREMENT is specified and there is no key that uses the field,
create a unique key over it.

This should also be done if auto_increment is not the first key part and the 
handler doesn't support this.
(In this case we should give a NOTE about this)