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/prefs_test.go

@@ -761,6 +761,42 @@ func TestMaskedPrefsPretty(t *testing.T) {
 			},
 			want: `MaskedPrefs{ExitNodeIP=100.102.104.105}`,
 		},
+		{
+			m: &MaskedPrefs{
+				Prefs: Prefs{
+					AutoUpdate: AutoUpdatePrefs{Check: true, Apply: false},
+				},
+				AutoUpdateSet: AutoUpdatePrefsMask{CheckSet: true, ApplySet: false},
+			},
+			want: `MaskedPrefs{AutoUpdate={Check=true}}`,
+		},
+		{
+			m: &MaskedPrefs{
+				Prefs: Prefs{
+					AutoUpdate: AutoUpdatePrefs{Check: true, Apply: true},
+				},
+				AutoUpdateSet: AutoUpdatePrefsMask{CheckSet: true, ApplySet: true},
+			},
+			want: `MaskedPrefs{AutoUpdate={Check=true Apply=true}}`,
+		},
+		{
+			m: &MaskedPrefs{
+				Prefs: Prefs{
+					AutoUpdate: AutoUpdatePrefs{Check: true, Apply: false},
+				},
+				AutoUpdateSet: AutoUpdatePrefsMask{CheckSet: false, ApplySet: true},
+			},
+			want: `MaskedPrefs{AutoUpdate={Apply=false}}`,
+		},
+		{
+			m: &MaskedPrefs{
+				Prefs: Prefs{
+					AutoUpdate: AutoUpdatePrefs{Check: true, Apply: true},
+				},
+				AutoUpdateSet: AutoUpdatePrefsMask{CheckSet: false, ApplySet: false},
+			},
+			want: `MaskedPrefs{}`,
+		},
 	}
 	for i, tt := range tests {
 		got := tt.m.Pretty()