core: Unit's condition_result field is a boolean

Let's only assign boolean values to a boolean variable.

Unit's condition_result is not of type ConditionResult, slightly
confusingly. Let's hence not assign one of ConditionResult's values to
it, but simple booleans.

This effectively doesn't make a difference, since CONDITION_ERROR is
true when cast to bool. But it's still ugly to rely on that. And
confusing.