util/slicesx: add EqualSameNil, like slices.Equal but same nilness

Then use it in tailcfg which had it duplicated a couple times.

I think we have it a few other places too.

And use slices.Equal in wgengine/router too. (found while looking for callers)

Updates #cleanup

Change-Id: If5350eee9b3ef071882a3db29a305081e4cd9d23
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/slicesx/slicesx_test.go

@@ -7,6 +7,8 @@ import (
 	"reflect"
 	"slices"
 	"testing"
+
+	qt "github.com/frankban/quicktest"
 )
 
 func TestInterleave(t *testing.T) {
@@ -84,3 +86,14 @@ func TestPartition(t *testing.T) {
 		t.Errorf("odds: got %v, want %v", odds, wantOdds)
 	}
 }
+
+func TestEqualSameNil(t *testing.T) {
+	c := qt.New(t)
+	c.Check(EqualSameNil([]string{"a"}, []string{"a"}), qt.Equals, true)
+	c.Check(EqualSameNil([]string{"a"}, []string{"b"}), qt.Equals, false)
+	c.Check(EqualSameNil([]string{"a"}, []string{}), qt.Equals, false)
+	c.Check(EqualSameNil([]string{}, []string{}), qt.Equals, true)
+	c.Check(EqualSameNil(nil, []string{}), qt.Equals, false)
+	c.Check(EqualSameNil([]string{}, nil), qt.Equals, false)
+	c.Check(EqualSameNil[[]string](nil, nil), qt.Equals, true)
+}