Add definitions for "up" and "down" commands that print/display the current

Python file/line when the current C execution frame is inside
PyEval_EvalFrame.  These are commented out by default because GDB sometimes
crashes as a result (seems like a GDB bug).

Add a pyframe command that displays the current Python stack frame.  If the
marked lines are uncommented, it will also cause Emacs/XEmacs to display the
current file/line.