WS client: Pass user_data also as callback argument
WS client: Pass user_data also as callback argument

Currently the user_data argument supplied to mg_connect_websocket_client
is stored in the context only, and accessible using mg_get_context and
mg_get_user_data. It should also be passed directly to the data_func
and close_func callbacks.

See #409

GerHobbelt/civet-webserver,GerHobbelt/civet-webserver,GerHobbelt/civet-webserver,GerHobbelt/civet-webserver,GerHobbelt/civet-webserver,GerHobbelt/civet-webserver