WireGuard-Go Android support

["Aurélien Chabot" <aurelien@chabot.fr>]

Hi,

This is a set of patch the wireguard-go project adding some needed stuff
to have android support. 
The idea is to use the wireguard-go project as a library that is exposed
with a "gomobile" binding to an android application.
I posted a set of patch on the wireguard-android project too and it is
worth for anyone looking at those change to have a look there too, at
least at the couple of go file that live there.
The change look a bit intrusive here (I had to move most of the code to
a folder...) but I did not find any other clean solution to be able to
use this code as an outside library (although I am not a go expert). I
am open to any better idea.

Regarding the change itself, there is one item that i'd like to
highlight, it's the change on that add a "tun device close". I am not
sure why I had to add this code, maybe I miss something else more
general, but it was actually never called in my case.   

Aurélien