mosquitto_broker.h and mosquitto_plugin.h

Thanks to Wolfgang Petroschka.

Closes #1290.

Thanks to Karl Palsson.

Closes #1266.

Thanks to Erik Moqvist.

Closes #1274.

Thanks to jsaak.

Closes #1276.

Thanks to TimmvonderMehden.

Closes #1264.

* Fix Will message for a persistent client incorrectly being sent when the client reconnects after a clean disconnect.
* Fix Will message for a persistent client not being sent on disconnect.
* Fix mosquitto_pub not using the `-c` option.

Thanks to Yannic Schröder.

Closes #1273.

Thanks to janniswarnat.

Closes #1285.

Thanks to Rob de Jonge.

Closes #1284.

This was not being copied to the in-use listener when compiled without
TLS support.

Fixed Segmentation Fault / NULLptr dereference

Signed-off-by: Till Zimmermann <tzimmermann@uni-osnabrueck.de>

Closes #1263. Thanks to vrst37.

Close #1245. Thanks to Christoph Krey.

This occurred when `check_retain_source` was set to true.

Closes #1245. Thanks to Christoph Krey.

Closes #1248. Thanks to Andrew J Freyer.

This is part of the cleanup routine when the client is already exiting.

Closes #1246. Thanks to Christoph Krey.

The Will message has as its first property one of content-type,
correlation-data, payload-format-indicator, or response-topic.

Closes #1244. Thanks to Christoph Krey.

Closes #1250. Thanks to bmparr.

Closes #1049. Thanks to Greg Troxel.

Closes #1048. Thanks to Greg Troxel.

- Split CFLAGS and CPPFLAGS, and LDFLAGS and LDADD/LIBADD.
- test/unit now respects CPPFLAGS and LDFLAGS.

Closes #1232. Thanks to Greg Troxel.

Closes #1231. Thanks to Greg Troxel.

Also support test target in cmake.