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>

net/udprelay/status/status.go

@@ -14,8 +14,9 @@ import (
 type ServerStatus struct {
 	// UDPPort is the UDP port number that the peer relay server forwards over,
 	// as configured by the user with 'tailscale set --relay-server-port=<PORT>'.
-	// If the port has not been configured, UDPPort will be nil.
-	UDPPort *int
+	// If the port has not been configured, UDPPort will be nil. A non-nil zero
+	// value signifies the user has opted for a random unused port.
+	UDPPort *uint16
 	// Sessions is a slice of detailed status information about each peer
 	// relay session that this node's peer relay server is involved with. It
 	// may be empty.