[MI] Synchronous event notification

[Andrew Cagney]

> UIs prefer to respond to events when something happens. In the later case
> above, we've asked gdb to install a breakpoint at main, but the UI won't
> actually do anything (like update breakpoint markers or breakpoint
> windows) UNTILL it gets the event from gdb.
> 
> So, if we need to vote, I vote that we output aync events all the time.
> (Of course, this would mean we really need to get target output nailed
> for the natives, but that's another battle.)

I don't think a vote is needed - it matches the original MI intent.  All 
events were to be considered considered async.

Andrew