This is another fallback needed on Win95, where the Win32 API
functions to convert between multibyte and wide strings exist, but
they haven't heard of the UTF-8 code page. PuTTY can't really do
without that these days.
(In particular, if a server sends a remote window-title escape
sequence while the terminal is in UTF-8 mode, then _something_ needs
to translate the UTF-8 data into Unicode for Windows to reconvert into
the character set used in window titles.)
This is a weird enough thing to be doing that I've put it under the
new #ifdef LEGACY_WINDOWS, so behaviour in the standard builds should
be unchanged.
This gets rid of all those annoying 'win', 'ux' and 'gtk' prefixes
which made filenames annoying to type and to tab-complete. Also, as
with my other recent renaming sprees, I've taken the opportunity to
expand and clarify some of the names so that they're not such cryptic
abbreviations.
