The function to convert UTF-16 to UTF-8 was only allocating a
buffer of wcslen(utf16str) bytes for the UTF-8 string, but that
is not sufficient if you have multibyte characters, and so when
those occured, the conversion was failing. This updates the
conversion functions to use the Win APIs to calculate the correct
buffer lengths.
Also fixes a comparison in the unit tests that would fail if
you did not have a particular environment variable set.
On Windows, we are having problems with home directories
that have non-ascii characters in them. This rewrites the
relevant code to fetch environment variables as UTF-16 and
then explicitly map then into UTF-8 for our internal usage.