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/ipn_clone.go

@@ -102,7 +102,7 @@ var _PrefsCloneNeedsRegeneration = Prefs(struct {
 	PostureChecking            bool
 	NetfilterKind              string
 	DriveShares                []*drive.Share
-	RelayServerPort            *int
+	RelayServerPort            *uint16
 	RelayServerStaticEndpoints []netip.AddrPort
 	AllowSingleHosts           marshalAsTrueInJSON
 	Persist                    *persist.Persist