tailcfg, feature/relayserver, wgengine/magicsock: invert UDP relay server nodeAttrs (#16444)

Inverts the nodeAttrs related to UDP relay client/server enablement to disablement, and fixes up the corresponding logic that uses them. Also updates the doc comments on both nodeAttrs.

Fixes tailscale/corp#30024

Signed-off-by: Dylan Bargatze <dylan@tailscale.com>

wgengine/magicsock/magicsock.go

@@ -2703,7 +2703,7 @@ func (c *Conn) onNodeViewsUpdate(update NodeViewsUpdate) {
 	peersChanged := c.updateNodes(update)
 
 	relayClientEnabled := update.SelfNode.Valid() &&
-		update.SelfNode.HasCap(tailcfg.NodeAttrRelayClient) &&
+		!update.SelfNode.HasCap(tailcfg.NodeAttrDisableRelayClient) &&
 		envknob.UseWIPCode()
 
 	c.mu.Lock()

wgengine/magicsock/magicsock_test.go

@@ -3408,9 +3408,6 @@ func Test_peerAPIIfCandidateRelayServer(t *testing.T) {
 	}
 	peerOnlyIPv4 := &tailcfg.Node{
 		Cap: math.MinInt32,
-		CapMap: map[tailcfg.NodeCapability][]tailcfg.RawMessage{
-			tailcfg.NodeAttrRelayServer: nil,
-		},
 		Addresses: []netip.Prefix{
 			netip.MustParsePrefix("2.2.2.2/32"),
 		},