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.

src/core/unit.c

@@ -1727,7 +1727,7 @@ static bool unit_test_condition(Unit *u) {
         r = manager_get_effective_environment(u->manager, &env);
         if (r < 0) {
                 log_unit_error_errno(u, r, "Failed to determine effective environment: %m");
-                u->condition_result = CONDITION_ERROR;
+                u->condition_result = true;
         } else
                 u->condition_result = condition_test_list(
                                 u->conditions,