Add endpoint tracking to the stack.

In the future this will replace DanglingEndpoints. DanglingEndpoints must be
kept for now due to issues with save/restore.

This is arguably a cleaner design and allows the stack to know which transport
endpoints might still be using its link endpoints.

Updates #837

PiperOrigin-RevId: 277386633

pkg/sentry/socket/netstack/stack.go

@@ -291,3 +291,18 @@ func (s *Stack) FillDefaultIPTables() {
 func (s *Stack) Resume() {
 	s.Stack.Resume()
 }
+
+// RegisteredEndpoints implements inet.Stack.RegisteredEndpoints.
+func (s *Stack) RegisteredEndpoints() []stack.TransportEndpoint {
+	return s.Stack.RegisteredEndpoints()
+}
+
+// CleanupEndpoints implements inet.Stack.CleanupEndpoints.
+func (s *Stack) CleanupEndpoints() []stack.TransportEndpoint {
+	return s.Stack.CleanupEndpoints()
+}
+
+// RestoreCleanupEndpoints implements inet.Stack.RestoreCleanupEndpoints.
+func (s *Stack) RestoreCleanupEndpoints(es []stack.TransportEndpoint) {
+	s.Stack.RestoreCleanupEndpoints(es)
+}