Make `camcontrol modepage` support block descriptors.

Make `camcontrol modepage` support block descriptors.

It allows to read and write block descriptors alike to mode page parameters.
It allows to change block size or short-stroke HDDs or overprovision SSDs.
Depenting on -P parameter the change can be either persistent or till reset.
In case of block size change device may need reformat after the setting.
In case of SSD overprovisioning format or sanitize may be needed to really
free the flash.

During implementation appeared that csio_encode_visit() can not handle
integers of more then 4 bytes, that makes 8-byte LBA handling awkward.
I had to split it into two 4-byte halves now.

MFC after:	1 week
Relnotes:	yes
Sponsored by:	iXsystems, Inc.