enum: Return boolean result for enum_from_name() lookup

Handling the result for enum_from_name() is difficult, as checking for
negative return values requires a cast if the enum type is unsigned. The new
signature clearly differentiates lookup result from lookup value.

Further, this actually allows to convert real -1 enum values, which could not
be distinguished from "not-found" and the -1 return value.

This also fixes several clang warnings where enums are unsigned.

src/conftest/hooks/unsort_message.c

@@ -69,8 +69,7 @@ METHOD(listener_t, message, bool,
 		order = enumerator_create_token(this->order, ", ", " ");
 		while (order->enumerate(order, &name))
 		{
-			type = enum_from_name(payload_type_short_names, name);
-			if (type != -1)
+			if (enum_from_name(payload_type_short_names, name, &type))
 			{
 				enumerator = list->create_enumerator(list);
 				while (enumerator->enumerate(enumerator, &payload))