wireguard-windows incompatible with Win10 hotspot

[Jason A. Donenfeld]

Hi lsc,

Thanks for the report. That's a curious bug indeed, especially given
that it affects both implementations.

For the wireguard-go/wintun implementation, we're using
setsockopt(IP_UNICAST_IF). For the wireguard-nt/kernel implementation,
we're using IP_PKTINFO in a cmsghdr. Judging by the description of
your report, it sounds like both of these cases are being ignored for
some reason when hotspot mode is enabled. Having a functional
IP_UNICAST_IF, and moreover a functional IP_PKTINFO, is extremely
important for things to work properly, and without it you wind up with
routing loops like the one you've described. It's actually not just
wireguard that uses those too -- I was reading the msquic source last
night and noticed that it does the same.

So, hm. I'll have to try to make a test environment for that, though
it might be slightly tricky given the hardware I have available. I'll
see what I can do.

Jason