wgengine,magicsock: fix two lazy wireguard config issues

1) we weren't waking up a discoEndpoint that once existed and
   went idle for 5 minutes and then got a disco message again.

2) userspaceEngine.noteReceiveActivity had a buggy check; fixed
   and added a test

wgengine/magicsock/magicsock_test.go

@@ -22,6 +22,7 @@ import (
 	"sync"
 	"testing"
 	"time"
+	"unsafe"
 
 	"github.com/google/go-cmp/cmp"
 	"github.com/tailscale/wireguard-go/device"
@@ -1322,3 +1323,17 @@ func stringifyConfig(cfg wgcfg.Config) string {
 	}
 	return string(j)
 }
+
+func TestDiscoEndpointAlignment(t *testing.T) {
+	var de discoEndpoint
+	off := unsafe.Offsetof(de.lastRecvUnixAtomic)
+	if off%8 != 0 {
+		t.Fatalf("lastRecvUnixAtomic is not 8-byte aligned")
+	}
+	if !de.isFirstRecvActivityInAwhile() { // verify this doesn't panic on 32-bit
+		t.Error("expected true")
+	}
+	if de.isFirstRecvActivityInAwhile() {
+		t.Error("expected false on second call")
+	}
+}