Do not expose the __name__ when reporting the list of options available
for a section since that is for internal use.
This closes SourceForge bug #115357.
Additionally, define InterpolationDepthError and MAX_INTERPOLATION_DEPTH.
The exception is raised by get*() when value interpolation cannot be
completed within the defined recursion limit. The constant is only
informative; changing it will not affect the allowed depth.
Fix the exit from get() so that None is not returned if the depth is met
or exceeded; either return the value of raise InterpolationDepthError.