tools: add dvpn

dvpn tool is a VPN based on the DHTNET stack. The tool creates a TUN interface to efficiently route traffic to the destination.

For the client side, upon establishing a connection, the tool creates a TUN interface and sets it as the default route in the routing table, excluding traffic destined for the peer. On the server side, a TUN interface is created, and NAT configuration is used to respond to client requests.

Change-Id: I43ff43982930d97502a64d15aeb2c8df283bdda9
GitLab: #13