minimal: add helpers and adapt all users

lib/libwebsockets.h

...les/client-server/minimal-ws-proxy/protocol_lws_minimal.c

minimal-examples/client-ws/README.md

...al-examples/client-ws/minimal-ws-client-rx/CMakeLists.txt

minimal-examples/client-ws/minimal-ws-client-rx/README.md

...amples/client-ws/minimal-ws-client-rx/minimal-ws-client.c

...amples/server-ws/minimal-ws-broker/protocol_lws_minimal.c

...-examples/server-ws/minimal-ws-server-ring/CMakeLists.txt

...s/server-ws/minimal-ws-server-ring/protocol_lws_minimal.c

...erver-ws/minimal-ws-server-threads/protocol_lws_minimal.c

plugins/protocol_lws_mirror.c