ipn/ipnlocal: make applySysPolicy a method on LocalBackend

Now that applySysPolicy is only called by (*LocalBackend).reconcilePrefsLocked,
we can make it a method to avoid passing state via parameters and to support
future extensibility.

Also factor out exit node-specific logic into applyExitNodeSysPolicyLocked.

Updates tailscale/corp#29969

Signed-off-by: Nick Khyl <nickk@tailscale.com>

ipn/ipnlocal/local_test.go

@@ -2968,7 +2968,8 @@ func TestApplySysPolicy(t *testing.T) {
 			t.Run("unit", func(t *testing.T) {
 				prefs := tt.prefs.Clone()
 
-				gotAnyChange := applySysPolicy(prefs, false)
+				lb := newTestLocalBackend(t)
+				gotAnyChange := lb.applySysPolicyLocked(prefs)
 
 				if gotAnyChange && prefs.Equals(&tt.prefs) {
 					t.Errorf("anyChange but prefs is unchanged: %v", prefs.Pretty())
@@ -3116,7 +3117,8 @@ func TestPreferencePolicyInfo(t *testing.T) {
 					prefs := defaultPrefs.AsStruct()
 					pp.set(prefs, tt.initialValue)
 
-					gotAnyChange := applySysPolicy(prefs, false)
+					lb := newTestLocalBackend(t)
+					gotAnyChange := lb.applySysPolicyLocked(prefs)
 
 					if gotAnyChange != tt.wantChange {
 						t.Errorf("anyChange=%v, want %v", gotAnyChange, tt.wantChange)