nbd: Add version 3.20

[donald]

Add user mode tools for nbd (nbd-server, nbd-client)

Note, that there is also a nbd server provided by qemu (qemu-nbd) which
is able to serve qcow2 formated files.

Usage example:

if nbd-client -c /dev/nbd0; then
    sudo nbd-client -d /dev/nbd0
fi
if test -e server.pid; then
    kill $(cat server.pid)
    rm server.pid
fi
test -e /scratch/local2/test.qcow && rm /scratch/local2/test.qcow
qemu-img create -f qcow2 /scratch/local2/test.qcow 80T
qemu-nbd --persistent /scratch/local2/test.qcow &
echo $! > server.pid
sudo nbd-client localhost /dev/nbd0
sudo mkfs.ext4 /dev/nbd0
sudo mount /dev/nbd0 /mnt

Create bee file with

cp scripts/mTEMPLATE.be0 nbd.be0

and fix BEE_VERSION, info URL and SRCURL.