Patch from Chris Boyle to fix Signpost's labelling when you have more

than 26 separate linked chains of unnumbered squares: we now wrap from
'z' to an Excel-like 'aa', 'ab', ..., instead of falling off z into
punctuation and control characters.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@9304 cda61777-01e9-0310-a592-d414129be87e