WireGuard root-less support for android

[Mathias Hall-Andersen]

Hi Aurélien

Thanks for contributing to the wireguard-go project.

I never anticipated for the implementation to be used as a library.
This means that I either have to:

1. Settle for an API and reconsider what is exported and not.
2. Give no guarantees about API stability

Providing the functionality as a library might be the cleanest solution
in this case.
One option is to have an internal package and a small exported API.

Is the GetUDPConn only used to wait for the device to bind?

The missing device.tun.device.Close() is indeed a bug.

I will look more at your patches during the weekend,
Thanks once again.

Mathias