Use the Unicode Converter to convert from Unicode to the display encoding

Use the Unicode Converter to convert from Unicode to the display encoding
if it's available.  Linking against the static Unicode Converter library
costs us about 30k on Classic 68K, which I can live with.

Because the default fallback converter can generate multiple output
characters for a single input character, we provide our own fallback that
doesn't.  It converts everything to '?' instead.

git-svn-id: svn://svn.tartarus.org/sgt/putty@2315 cda61777-01e9-0310-a592-d414129be87e