[RFA/JVMTI] SingleStep notification

[Keith Seitz]

Tom Tromey wrote:

> Keith> Any less offensive?
> 
> If you think this is a bad idea, feel free to push back :)

No reason to push back: you were absolutely right. The only disadvantage 
to this is if someone doesn't use the standard JVMTI AgentOnLoad type 
callback to create its jvmtiEnv. If someone does this down the line in a 
jni call, well, it's going to cause all kinds of havoc.

I'll commit this now, double-checking that this limitation is properly 
spelled out.

Thanks,
Keith