scripts/prestart.sh: don't execute fixup_udev in background

This script must be started before the init will be executed,
so it must not be executed in background.

Probably the author want to write && instead of &.
I don't think, that we need to check errors here,
because the next script is independent from the previous one.
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.