libwebsockets

freertos: lws_cancel_service udp as pipe

Freertos + lwip doesn't support pipe2() or pipe()... implement a "pipe"
based on two UDP sockets, one listening on 127.0.0.1:54321 and the other
doing a sendto() there of a single byte to interrupt the event loop wait.

Re-use the arrangements for actual pipe fds and pipe role to deliver
lws_cancel_service() functionality using this.