Make single bit bitfields unsigned to avoid clang 16 warning (#1860)

Make single bit bitfields unsigned to avoid clang 16 warning (#1860)

Clang 16 introduced a warning about single bit bitfields in structs,
which is triggered by a few libarchive formats:

libarchive/archive_write_set_format_7zip.c:1541:13: error: implicit
truncation from 'int' to a one-bit wide bit-field changes value from 1
to -1 [-Werror,-Wsingle-bit-bitfield-constant-conversion]
                    file->dir = 1;
                              ^ ~

This is because single bit bitfields only support values -1 and 0, if
they are signed.

For bitfields with two or more bits this can be intentional, but single
bit bitfields are typically used as booleans, so it is better to make
them unsigned.