add gnu screen
move root-perms functionality from stage1 to write-hd-image

applying the permissions in the chroot has 2 unwanted consequences:

1) once "super-chroot" [1] has been integrated, we want to be able to
   use the rootfs without root powers,
2) it's now possible to run qemu on the sabotage rootfs tree,
   however requiring to run qemu as root is not very appealing.

generally, it seems that it's not the job of package build scripts
to set permissions, but rather the final distribution's.

[1] http://openwall.com/lists/musl/2013/08/01/2
rc.boot: support rootfs usage without r/w access
add ttyS0 service, required for qemu console mode
add util to boot chroot with kernel directly in qemu

virtio over plan9 is used.
tested on x86_64.