compel: plugins -- Add shmem plugin

The shmem pluging allows creation of shared memory segment between
parasite code and the caller.

Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>