tar: Fix path encoding conversion error messages

Since commit 71c981a213 (Encoding: Fix potential encoding issues with
libarchive on Windows, 2014-06-12, v3.1.0-rc1~350^2) we use libarchive's
wide character APIs for entry paths.  This works well on Windows where
this conversion expects UTF-8 and produces UTF-16.  However, on
non-Windows platforms this conversion expects the locale's encoding.

Under `LANG=C` conversion may fail on paths using encodings such as
UTF-8, but we've been ignoring `ToWide` failure and passing empty paths
to libarchive.  This causes incomplete diagnostics like "Can't lstat".

Restore use of libarchive's narrow APIs on non-Windows platforms.  This
defers encoding conversions to libarchive, enabling better diagnostics
like "Can't translate Pathname '...' to UTF-8".

Issue: #26903