[linux] fix stepping over fork in follow-child mode.

[Joel Brobecker]

Hi Daniel,

> > Here's an alternative fix.  Vladimir, this is also the patch I was
> > talking about earlier on IRC.  Not tested or finished yet.
> 
> Any comments?  Otherwise I'll commit this in a few days.

I'm really amazed at the amount of simplification this brings.
Never having looked at thread support on Linux, I had no idea...

I only have one very minor question: Why is the thread exit notification
now conditional on print_thread_events? I did notice that you changed
the few printf_filtered of such events into printf_unfiltered the same
way we did for the new thread events, which is nice :).

Thanks,
-- 
Joel