Fix the archive_write_data() function so it always returns
number of bytes written, even when used to write files to
disk. Extend the test suite to verify the correct return
values for archive_write_data() and archive_write_data_block().

Thanks to: Bruce Mah, for stepping in promptly to back out the

earlier broken version of this fix

Thanks to: Colin Percival, for pointing out the correct fix
MFC after: 5 days
Approved by: re (ksmith)
Pointy hat: \me