Add a new Windows-only public API, archive_read_open_filenames_w (#2016)

Add a new Windows-only public API, archive_read_open_filenames_w (#2016)

There is a discrepancy between the w and non-w filename APIs, wherein a
consumer of libarchive can open a multi-volume set with names in the
current locale (on Windows) but not with UTF-16 names.

This patch addresses that issue.

archive_read_open_filename_w delegates its work to
archive_read_open_filenames_w.

Fixes #1728

Tested passing on Windows. In the meantime, I will also test on Linux.
I am hoping that the build agents can help me determine FreeBSD and
macOS coverage.