hook_ct: mount devtmpfs in CT

All modern distributions require devtmpfs in /dev. devtmpfs can't
be mounted from userns. This patch bind-mounts the host /dev.
It's secure, because permissions are handled according with uid and
gid maps for the user namespace.

This patch removes old hacks about devices. They are not required any more.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
prestart.sh: remove fixup_udev() completely

The whole point of fixup_udev() was to wait for container to be started
and then umount those mounts done from the inside of container, in order
to uncover the mounts previously done by vzctl.

Now, previous commit removes those mounts from vzctl code, as well
as umounts from this script. What is left in fixup_udev() is code
to wait for container to start. Remove that as well.

Cc: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Kir Kolyshkin <kir@openvz.org>