cancel_service: change to event-loop agnostic implementation

 - Add platform helpers for pipe creation.

 - Change the direct-to-fds implementation to create a wsi for each
   pt and use the normal apis to bind it to the event loop.

 - Modifiy context creation and destroy to create and remove the
   event pipe wsis.

 - Create the event pipe wsis during context create if using the
   default poll() event loop, or when the other event loops start
   otherwise.

 - Add handler that calls back user code with
   LWS_CALLBACK_EVENT_WAIT_CANCELLED

This patch allows you to call `lws_cancel_service(struct lws_context *context)`
from another thread.

It's very cheap for the other thread to call and is safe without
locking.

Every use protocol receives a LWS_CALLBACK_EVENT_WAIT_CANCELLED from
the main thread serialized normally in the event loop.