libwebsockets

ss: wire up EVENT_WAIT_CANCELLED

Currently the lws_cancel_service() api only manifests itself at lws level.
This adds a state LWSSSCS_EVENT_WAIT_CANCELLED that is broadcast to all
SS in the event loop getting the cancel service api call, and allows
SS-level user code to pick up handling events from other threads.

There's a new example minimal-secure-streams-threads which shows the
pattern for other threads to communicate with and trigger the event in the
lws service thread.