device: receive: do not exit immediately on transient UDP receive errors

Some users report seeing lines like:

> Routine: receive incoming IPv4 - stopped

Popping up unexpectedly. Let's sleep and try again before failing, and
also log the error, and perhaps we'll eventually understand this
situation better in future versions.

Because we have to distinguish between the socket being closed
explicitly and whatever error this is, we bump the module to require Go
1.16.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

conn/conn_linux.go

@@ -204,7 +204,7 @@ func (bind *nativeBind) ReceiveIPv6(buff []byte) (int, Endpoint, error) {
 
 	var end NativeEndpoint
 	if bind.sock6 == -1 {
-		return 0, nil, syscall.EAFNOSUPPORT
+		return 0, nil, net.ErrClosed
 	}
 	n, err := receive6(
 		bind.sock6,
@@ -220,7 +220,7 @@ func (bind *nativeBind) ReceiveIPv4(buff []byte) (int, Endpoint, error) {
 
 	var end NativeEndpoint
 	if bind.sock4 == -1 {
-		return 0, nil, syscall.EAFNOSUPPORT
+		return 0, nil, net.ErrClosed
 	}
 	n, err := receive4(
 		bind.sock4,
@@ -237,12 +237,12 @@ func (bind *nativeBind) Send(buff []byte, end Endpoint) error {
 	nend := end.(*NativeEndpoint)
 	if !nend.isV6 {
 		if bind.sock4 == -1 {
-			return syscall.EAFNOSUPPORT
+			return net.ErrClosed
 		}
 		return send4(bind.sock4, nend, buff)
 	} else {
 		if bind.sock6 == -1 {
-			return syscall.EAFNOSUPPORT
+			return net.ErrClosed
 		}
 		return send6(bind.sock6, nend, buff)
 	}