Add vsock support

[benzea]

We were thinking that having vsock support in dropbear could be useful when having VMs with dropbear. We are considering to use this for OpenWRT/Gluon testing specifically as we could just add a vsock listener by default and then use it in the environment. While on real HW, it would just fail to listen on that socket.

The approach here is to accept AF_VSOCK anywhere where IP sockets are also allowed. Not sure that is a great idea. I am happy to change the approach.

I have tested that listening on the server, connecting the client and port forwarding work.

Examples (just running on the host):

./dropbear -p localhost:22 -p %vsock:22 -E -F -R
./dbclient 1%vsock -p 22 -L %vsock:80:localhost:80