If for some reason do_text() fails to translate from Unicode to the font

If for some reason do_text() fails to translate from Unicode to the font
encoding, have it go through the rest of its motions with an empty string
anyway, so as to at least give a sensible empty box of the right colour.

If SetFallbackUnicodeToText() fails, switch over to using the charset
library, hence avoiding problems in do_text().

If the version of the Unicode Converter we're using doesn't understand about
interrupt-safe fallback functions, don't try to tell it we've got one.  This
prevents SetFallbackUnicodeToText() from failing on systems with old Unicode
Converters.

git-svn-id: http://svn.tartarus.org/sgt/putty@2414 cda61777-01e9-0310-a592-d414129be87e