gdb.texinfo: Separate the index into 2 indices, cp and fn, to

 gdb.texinfo: Separate the index into 2 indices, cp and fn, to
 avoid file-name clashes between Index.html and index.html on
 case-insensitive filesystems.  See the discussion starting in
 http://sourceware.org/ml/gdb-patches/2012-06/msg00457.html for the
 details of the problem.
 (Top): Update the master menu for the above.
 (Concept Index, Command and Variable Index): New nodes, instead of
 the old "Index" node that was deleted.  All cross-references
 updated.