Remove global variables

Remove global variables used for passing context & call stack
information. The users of the global variables are adjusted accordingly
to use public functions.
Refer to PyKd frames as 'raw' frames

When referring to PyKd frame objects, use 'raw' instead of 'native' to
avoid ambiguities.
Show native frames until first X++ frame

When displaying the call stack, show native frames until the first X++
frame is found. This provides more information about the crash but still
avoids overwhelming the investigator with unnecessary native frames.
Make some non-public functions semi-private

Clean up the public interface of the call stack module by making some of
the non-public functions semi-private.