Release 1.5.3

Security:

• Fix CVE-2018-12543. If a message is sent to Mosquitto with a topic that begins with $, but is not $SYS, then an assert that should be unreachable is triggered and Mosquitto will exit.

Broker:

• Elevate log level to warning for situation when socket limit is hit.
• Remove requirement to use user root in snap package config files.
• Fix retained messages not sent by bridges on outgoing topics at the first connection.
• Documentation fixes.
• Fix duplicate clients being added to by_id hash before the old client was removed.
• Fix Windows version not starting if include_dir did not contain any files.

Build:

• Various fixes to ease building.

Use WITH_BUNDLED_DEPS=no