Remove undefined functions superseded by Strutil.

Strutil::utf8_to_utf16 and Strutil::utf16_to_utf8 replaced path_to_windows_native and
path_from_windows_native respectively.
Fix MSVC cast warnings.
Explicitly convert to UTF8 native arguments in each tool.

An ArgParse instance does not necessarily use the process command line
parameters, it is only the original arguments in the "main" entry
point which need to be converted. Otherwise custom command lines, as used
by oiiotool, would be replaced by the one used to create the process.
Do not rely on the shell to expand wildcards for the oiiotool test.

This fixes the test for Windows, where unlike bash the shell does not expand wildards.