With this change, I'm able to connect to a wireguard peer by running
	doas wg-quick up /usr/local/etc/wireguard/wg0.conf
where the content of wg0.conf, with some parts censored as "XXX", is:
	[Interface]
	PrivateKey = XXX
	Address=10.167.1.5

	[Peer]
	PublicKey = XXX
	EndPoint = exoco.falsifian.org:791
	AllowedIps = 10.167.1.0/24
	PersistentKeepalive = 25

Essentially, there are three changes, all of which will hopefully
appear upstream eventually:

1. Fix DragonflyBSD support in the x/net go library.

   This change is under review upstream at
   https://go-review.googlesource.com/c/net/+/328331/

   Here, I'm actually patching the vendored copy of x/net used by
   lang/go and lang/go-devel, because the problem manefests via the
   "net" library, which is provided directly by go (or go-devel).
   (Specifically, net.InterfaceByName doesn't work without this patch.)

2. Fix wireguard-go's DragonflyBSD support.

   This change, originally by Aaron Li, is under review at
   https://github.com/WireGuard/wireguard-go/pull/49

   I added GO_ENV to MAKE_ENV to fix wireguard-go vendoring, based on
   commit 7a2ae184 from FreeBSD ports.

3. A small fix to net/wireguard to be compatible with the newer
   wireguard-go.

   This change is cherry-picked from upstream net/wireguard.