- repair the _enable_single_crit method, it was named the same

- repair the _enable_single_crit method, it was named the same
as the attribute and probably just replaced itself, so that is
now _set_enable_single_crit
- as a side effect of the main issue fixed here, correct the case in
adjust_for_single_inheritance where the same mapper appears more
than once in mapper_adapter_map; run through a set() for uniqueness.
- Fixed bug in subquery eager loading in conjunction with
:func:`.with_polymorphic`, the targeting of entities and columns
in the subquery load has been made more accurate with respect
to this type of entity and others. Fixes #3106