tool: support UTF-16 command line on Windows

- use `wmain` instead of `main` when `_UNICODE` is defined [0]
- define `argv_item_t` as `wchar_t *` in this case
- use the curl_multibyte gear to convert the command-line arguments to
  UTF-8

This makes it possible to pass parameters with characters outside of
the current locale on Windows, which is required for some tests, e.g.
the IDN tests. Out of the box, this currently only works with the
Visual Studio project files, which default to Unicode, and winbuild
with the `ENABLE_UNICODE` option.

[0] https://devblogs.microsoft.com/oldnewthing/?p=40643

Ref: https://github.com/curl/curl/issues/3747
Closes https://github.com/curl/curl/pull/3784

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