Right now the contents of the sysvipc shmem segment is stored as
raw blob in the ipcns-shm image. This is bad for two reasons.

First is that huge segments with holes are filled with real zeroes
in image. Second is that we prefer not to mix images with pb object
and raw data.

To fix both let's re-use the shmem.c code that keeps anon shared
memory contents in pagemap/pages images.

https://github.com/xemul/criu/issues/287Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>