Allow spaces in section names.

Allow spaces in section names.
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.