ipn,cmd/tailscale/cli: support hierarchical MaskedPrefs (#10507)

Some fields if `ipn.Prefs` are structs. `ipn.MaskedPrefs` has a single
level of boolean `*Set` flags, which doesn't map well to nested structs
within `ipn.Prefs`.

Change `MaskedPrefs` and `ApplyEdits` to support `FooSet` struct fields
that map to a nested struct of `ipn.Prefs` like `AutoUpdates`. Each
struct field in `MaskedPrefs` is just a bundle of more `Set` bool fields
or other structs. This allows you to have a `Set` flag for any
arbitrarily-nested field of `ipn.Prefs`.

Also, make `ApplyEdits` match fields between `Prefs` and `MaskedPrefs`
by name instead of order, to make it a bit less finicky. It's probably
slower but `ipn.ApplyEdits` should not be in any hot path.

As a result, `AutoUpdate.Check` and `AutoUpdate.Apply` fields don't
clobber each other when set individually.

Updates #16247

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

cmd/tailscale/cli/cli_test.go

@@ -786,7 +786,7 @@ func TestPrefFlagMapping(t *testing.T) {
 	prefHasFlag := map[string]bool{}
 	for _, pv := range prefsOfFlag {
 		for _, pref := range pv {
-			prefHasFlag[pref] = true
+			prefHasFlag[strings.Split(pref, ".")[0]] = true
 		}
 	}