mqtt plugin: Don't use <stdbool.h>.

Instead, use the standard _Bool type.
mqtt plugin: Use the thread-safe sstrerror().
mqtt plugin: Correctly check the return value of format_topic().
mqtt plugin: Add support for multiple brokers.

Also adds support for authentication and configuring a couple of settings
(QoS, rates, retention).
mqtt plugin: Let "client ID" default to the hostname.

The MQTT documentation says that this needs to be unique and stable, so
I think the hostname is the best match for this.
mqtt plugin: Send only the acutally used part of the payload buffer.