wgengine/...: split into multiple receive functions

Upstream wireguard-go has changed its receive model.
NewDevice now accepts a conn.Bind interface.

The conn.Bind is stateless; magicsock.Conns are stateful.
To work around this, we add a connBind type that supports
cheap teardown and bring-up, backed by a Conn.

The new conn.Bind allows us to specify a set of receive functions,
rather than having to shoehorn everything into ReceiveIPv4 and ReceiveIPv6.
This lets us plumbing DERP messages directly into wireguard-go,
instead of having to mux them via ReceiveIPv4.

One consequence of the new conn.Bind layer is that
closing the wireguard-go device is now indistinguishable
from the routine bring-up and tear-down normally experienced
by a conn.Bind. We thus have to explicitly close the magicsock.Conn
when the close the wireguard-go device.

One downside of this change is that we are reliant on wireguard-go
to call receiveDERP to process DERP messages. This is fine for now,
but is perhaps something we should fix in the future.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

wgengine/userspace.go

@@ -320,8 +320,6 @@ func NewUserspaceEngine(logf logger.Logf, conf Config) (_ Engine, reterr error)
 				logf("[unexpected] peer %s has no single-IP routes: %v", peerWGKey.ShortString(), allowedIPs)
 			}
 		},
-		CreateBind:     e.magicConn.CreateBind,
-		CreateEndpoint: e.magicConn.CreateEndpoint,
 	}
 
 	e.tundev.OnTSMPPongReceived = func(pong packet.TSMPPongReply) {
@@ -336,8 +334,13 @@ func NewUserspaceEngine(logf logger.Logf, conf Config) (_ Engine, reterr error)
 
 	// wgdev takes ownership of tundev, will close it when closed.
 	e.logf("Creating wireguard device...")
-	e.wgdev = device.NewDevice(e.tundev, e.wgLogger.DeviceLogger, opts)
+	e.wgdev = device.NewDevice(e.tundev, e.magicConn.Bind(), e.wgLogger.DeviceLogger, opts)
 	closePool.addFunc(e.wgdev.Close)
+	closePool.addFunc(func() {
+		if err := e.magicConn.Close(); err != nil {
+			e.logf("error closing magicconn: %v", err)
+		}
+	})
 
 	go func() {
 		up := false