Support in the cross-platform code for translating to and from

UTF-16 when exchanging wchar_t strings with the front end. Enabled
by a #define in the platform's header file (one should not
promiscuously translate UTF-16 surrogate pairs on 32-bit wchar_t
platforms since that could give rise to redundant encoding attacks),
which is present on Windows.

git-svn-id: svn://svn.tartarus.org/sgt/putty@8495 cda61777-01e9-0310-a592-d414129be87e