from the private archive_write structure and fix up all writers to use
the format fields in the base "archive" structure. This error made it
impossible to query the format after setting up a writer because the
write format was stored in an inaccessible place.
In particular, the previous code led to archives that had
non-empty bodies following directory entries. Not a fatal
problem, as bsdtar and GNU cpio are both happy to just skip
this bogus data, but it still shouldn't be there.
MFC after: 3 days
