Turn off "Can't translate ... to UTF-8" warnings

[cperciva]

Libarchive, at https://github.com/Tarsnap/tarsnap/blob/master/libarchive/archive_write_set_format_pax.c#L489 and following, checks if paths and user/group names can be converted; if not, they're stored raw using the hdrcharset="BINARY" option.

Libarchive prints warnings when it does this, but these warnings are annoying and pointless since we're never going to extract with a tool which doesn't understand the pax options.

We should turn this off -- possibly with an option to re-enable the warnings if anyone likes seeing them.