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>

ipn/conf.go

@@ -124,7 +124,7 @@ func (c *ConfigVAlpha) ToPrefs() (MaskedPrefs, error) {
 	}
 	if c.AutoUpdate != nil {
 		mp.AutoUpdate = *c.AutoUpdate
-		mp.AutoUpdateSet = true
+		mp.AutoUpdateSet = AutoUpdatePrefsMask{ApplySet: true, CheckSet: true}
 	}
 	return mp, nil
 }