Change texindex sort order to two level case insensitive/sensitive

* texindex/texindex.awk (string_compare)
(string_compare_ignorecase, string_compare_noignorecase)
(string_compare_internal):
Add functions beyond string_compare.  Replace character-by-character
logic with two-pass comparison.  The first is case insensitive, the
second is case sensitive.

* texindex/ti.texi: add test cases and try to explain the change.