VPN improvements

[jdknives]

Feature description

A tentative list of improvements that need to be added to the VPN apps.

• add e2e encryption

• add authentication via passcode

• add RPC methods (change VPN server remote Pubkey, change password auth)

• allow to use VPN client as local access point

• add Windows client (depends on Build and run on Windows #343)

• move on to the ip command set for Linux systems - described in here VPN apps #278 (comment)

• add README.md

• there's a piece of code which searches for the default network interface. currently is invoked only on Mac systems. it works in most cases but not in all of them. code is unreliable and should be replaced