Add maximum-qos support to broker and client.

This comes in the form of:

* Per listener maximum_qos option, which can be in the range 0-2.
* Changes to mosquitto_publish*() to return MOSQ_ERR_QOS_NOT_SUPPORTED
  if attempting to publish with a higher QoS than supported.
* Bridges will downgrade messages to match the maximum QoS.

More tests on the broker side (specifically bridges) are required. This
needs bridge support for MQTT 5 first.