mqtt: fix MQTT client PINGREQ failure when using LIBUV as event driver.

[baldwin-law]

When libuv is used as the underlying event-driven mechanism, the keepalive operation between the MQTT client and the server fails. The reason is that the keepalive operation should send a PINGREQ, but in fact, it is not sent. The problem lies in the fact that the modification of the underlying I-O-related events is not triggered, resulting in the inability to trigger I/O events. Through testing, modifying the mux_substream flag can solve this problem.

[lws-team]

Thanks... does it make trouble to change the test to a runtime one like if (lws_check_opt(context->options, LWS_SERVER_OPTION_LIBUV)) ? The reason is it's possible to build lws with the LIBUV config, but actually select some other event loop at runtime.

[baldwin-law]

Of course, your approach is more elegant. I'll push the changes.