cmd/tailscale,ipn: add relay-server-port "tailscale set" flag and Prefs field (#15594)

This flag is currently no-op and hidden. The flag does round trip
through the related pref. Subsequent commits will tie them to
net/udprelay.Server. There is no corresponding "tailscale up" flag,
enabling/disabling of the relay server will only be supported via
"tailscale set".

This is a string flag in order to support disablement via empty string
as a port value of 0 means "enable the server and listen on a random
unused port". Disablement via empty string also follows existing flag
convention, e.g. advertise-routes.

Early internal discussions settled on "tailscale set --relay="<port>",
but the author felt this was too ambiguous around client vs server, and
may cause confusion in the future if we add related flags.

Updates tailscale/corp#27502

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

ipn/prefs_test.go

@@ -65,6 +65,7 @@ func TestPrefsEqual(t *testing.T) {
 		"PostureChecking",
 		"NetfilterKind",
 		"DriveShares",
+		"RelayServerPort",
 		"AllowSingleHosts",
 		"Persist",
 	}
@@ -73,6 +74,9 @@ func TestPrefsEqual(t *testing.T) {
 			have, prefsHandles)
 	}
 
+	relayServerPort := func(port int) *int {
+		return &port
+	}
 	nets := func(strs ...string) (ns []netip.Prefix) {
 		for _, s := range strs {
 			n, err := netip.ParsePrefix(s)
@@ -341,6 +345,16 @@ func TestPrefsEqual(t *testing.T) {
 			&Prefs{AdvertiseServices: []string{"svc:tux", "svc:amelie"}},
 			false,
 		},
+		{
+			&Prefs{RelayServerPort: relayServerPort(0)},
+			&Prefs{RelayServerPort: nil},
+			false,
+		},
+		{
+			&Prefs{RelayServerPort: relayServerPort(0)},
+			&Prefs{RelayServerPort: relayServerPort(1)},
+			false,
+		},
 	}
 	for i, tt := range tests {
 		got := tt.a.Equals(tt.b)