restorer: unmap itself (v2)

This patch adds a function for removing the restorer blob. This function
never returns and the process must be trapped on the exit from the
munmap syscall.

v2: * release parasite_ctl sturcture and use the new interface of
      parasite_prep_ctl

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>