This patches fixes a couple of left over instances of !is_running to use
is_stopped instead, and adds a missing check for is_exited in
get_selected_block, which I tripped on while development the yet
unsubmitted non-stop remote target.

I'm adding more comments to gdbthread.h to describe why.  Basically,
the thread state is not a boolean anymore.

I'll soon post a description of these states in the internal manual
under the currently empty section 3.5 - Thread Handling.  Let me
know if it should be placed somewhere else.

Tested on x86_64-unknown-linux-gnu sync and async.

OK?

-- 
Pedro Alves