Identifying a dummy frame using a frame id

[Andrew Cagney]

> - get_dummy_frame_id (target registers)
> Given a target stopped on that dummy frame breakpoint, return a frame ID that exactly matches dummy_frame_id() above.

Refinement #1.

unwind dummy frame id (next_frame)

Given the `next' frame, use frame_register_unwind() to construct the 
dummy frame's ID.

Why?  Because this also needs to work when the dummy-frame isn't 
innermost (the inner most frame edge case is covered by passing it the 
sentinel frame).

(my brain hurts!)
Andrew