[ddbg] ps should print source of current frame

[Bill Baxter]

Thanks for adding the 'ps' previously.  Makes debugging from the command 
line much much easier.

But I just noticed that it doesn't honor the current frame setting.  I 
think ps should print the source of the current location in the current 
frame, not the current location in the topmost frame.

This is how gdb and other debuggers I've used do it, and it makes sense 
I think.

--bb