mounts: handle shared and slave mounts (v3)

The idea is simple. If a mount can't be mounted now, we will try to
mount it later.

v2: don't wait slaves, they are unmounted anyway
v3: add a comment in do_bind_mount to explain restoring of shared groups
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>