[fitsbits] BINTABLE convention for >999 columns

[THIERRY FORVEILLE]

> From all the suggestions offered so far, Mark's is by far the most sensible
> in my opinion since it provides a significant expansion while preserving
> full backward compatibility.
> 
If the (mentioned but not yet fleshed out on the list) keyword length extension 
proposal also preserves backward compatibility, it would probably have my preference, 
as a more general solution which solves other problems as well.