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/libcharon/plugins/stroke/stroke_socket.c

@@ -584,8 +584,7 @@ static void stroke_loglevel(private_stroke_socket_t *this,
 	}
 	else
 	{
-		group = enum_from_name(debug_names, msg->loglevel.type);
-		if ((int)group < 0)
+		if (!enum_from_name(debug_names, msg->loglevel.type, &group))
 		{
 			fprintf(out, "unknown type '%s'!\n", msg->loglevel.type);
 			return;