now the interface will properly indicate that the archive is not writable
when that is the case (it was not even before the port to KArchive)
Signed-off-by: Ivailo Monev <xakepa10@gmail.com>
make it protected so that it is reusable and implement permissions
for LibArchiveInterface via it while at it
Signed-off-by: Ivailo Monev <xakepa10@gmail.com>