wgengine/router,ipn/ipnlocal: add MTU field to router config

The MacOS client can't set the MTU when creating the tun due to lack
of permissions, so add it to the router config and have MacOS set it
in the callback using a method that it does have permissions for.

Updates #8219

Signed-off-by: Val <valerie@tailscale.com>

wgengine/router/router_test.go

@@ -21,8 +21,8 @@ func mustCIDRs(ss ...string) []netip.Prefix {
 
 func TestConfigEqual(t *testing.T) {
 	testedFields := []string{
-		"LocalAddrs", "Routes", "LocalRoutes", "SubnetRoutes",
-		"SNATSubnetRoutes", "NetfilterMode",
+		"LocalAddrs", "Routes", "LocalRoutes", "NewMTU",
+		"SubnetRoutes", "SNATSubnetRoutes", "NetfilterMode",
 	}
 	configType := reflect.TypeOf(Config{})
 	configFields := []string{}
@@ -134,6 +134,16 @@ func TestConfigEqual(t *testing.T) {
 			&Config{NetfilterMode: preftype.NetfilterNoDivert},
 			true,
 		},
+		{
+			&Config{NewMTU: 0},
+			&Config{NewMTU: 0},
+			true,
+		},
+		{
+			&Config{NewMTU: 1280},
+			&Config{NewMTU: 0},
+			false,
+		},
 	}
 	for i, tt := range tests {
 		got := tt.a.Equal(tt.b)