cmd/tailscale/cli,ipn,all: make peer relay server port a *uint16

In preparation for exposing its configuration via ipn.ConfigVAlpha,
change {Masked}Prefs.RelayServerPort from *int to *uint16. This takes a
defensive stance against invalid inputs at JSON decode time.

'tailscale set --relay-server-port' is currently the only input to this
pref, and has always sanitized input to fit within a uint16.

Updates tailscale/corp#34591

Signed-off-by: Jordan Whited <jordan@tailscale.com>

ipn/prefs_test.go

@@ -78,7 +78,7 @@ func TestPrefsEqual(t *testing.T) {
 			have, prefsHandles)
 	}
 
-	relayServerPort := func(port int) *int {
+	relayServerPort := func(port uint16) *uint16 {
 		return &port
 	}
 	nets := func(strs ...string) (ns []netip.Prefix) {