mosquitto: major upgrade to 1.6.x

mosquitto: major upgrade to 1.6.x

Major new release of mosquitto.

This release rolls up the initial 1.6.0 release, plus the subsequent
build/bug fixes of 1.6.1 and 1.6.2.

Original upstream changelogs:
https://mosquitto.org/blog/2019/04/version-1-6-released/
https://mosquitto.org/blog/2019/04/version-1-6-1-released/
https://mosquitto.org/blog/2019/04/version-1-6-2-released/

Major features of interest:
* MQTTv5 support
* performance improvements
* ALPN support
* OCSP staping support
* OpenSSL Engine support
* TLSv1.0 support dropped

Currently adds two patches to continue supporting OpenSSL engine support
being disabled, and a missing header include.  These are both tracked
upstream and are expected to be dropped in a subsequent release.

Signed-off-by: Karl Palsson <redacted>