Remove PROP_UNDEF and PROP_ERROR in ClassAds internals.

Use EVAL_UNDEF and EVAL_ERROR instead, which are treated entirely the
same.
Add ClassAd::EvaluateAttrBoolEquiv(). #4785

Like EvaluateAttrBool(), but integers and reals will be converted to the
appropriate boolean value if old ClassAd semantics are enabled.
Finish coercion of numbers to booleans in new ClassAds. #4785

When we added the option to coerce integers and reals to boolean, we
missed a few spots.
Make ClassAd functions more robust to unexpected values.

When ClassAds functions get arguments with unexpected values, don't make
the evaluation fail. Just return a result of Error or other appropriate
value.