Integrating UPnP for auto port-forwarding

[slavko989]

UPnP/NAT-PMP support should be implemented so that on every node start it automatically determines the local IP address and via UPnP router/getaway support (all modern routers have it) forwards 40405 for TCP and UDP to that IP address on which the node is running.

This is a great way to potentially increase peer discovery.

Some programs, such as qBitTorrent already implement this, and every time you start them, they forward required ports via UPnP router feature, and when the program is closed those rules don't apply anymore - this kind of port forwarding isn't permanent of course.