Introduce the PS_IOV_ADD_F command that is to add pages with
flags. We already use the similar notation on page-xfer -- the
single write callback with pagemap and flags. For page-server
let's use the same. Legacy _HOLE and _PAGE handling is kept.

Changed commands numbers are OK, as the commands in question
are still in criu-dev branch.
Acked-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>