speed up msg box by caching the glist in t_messresponder for fetching $0
This brings msg box back to the same performance it had before $0 expansion was added as a feature to message boxes. The first implementation relied on calling canvas_setcurrent and canvas_unsetcurrent in the message box methods. This added a malloc/free for each message box method call. Instead this implementation caches the msg box glist in the t_messresponder. That is available as the target parameter in m_binbuf. Unfortunately, we have to add a parameter to binbuf_realizedollsym. Since that's a public function we have to break out the new param into binbuf_dorealizedollsym.