tablecmds.c/MergeAttributes() cleanup

The MergeAttributes() and related code in and around tablecmds.c is huge 
and ancient, with many things bolted on over time, and difficult to deal 
with.  I took some time to make careful incremental updates and 
refactorings to make the code easier to follow, more compact, and more 
modern in appearance.  I also found several pieces of obsolete code 
along the way.  This resulted in the attached long patch series.  Each 
patch tries to make a single change and can be considered incrementally. 
  At the end, the code is shorter, better factored, and I hope easier to 
understand.  There shouldn't be any change in behavior.