net: mqtt: Improve disconnect error notification for MQTT 5.0

As MQTT 5.0 allows to specify the disconnect reason in the Disconnect
packet, use this new feature to improve error notification to the
broker, according to the error guidelines in the MQTT 5.0 spec.
For most cases, a generic arbitrary mapping between errno values and
reason codes is used, however the parser can specify the disconnect
reason code manually to better handle certain corner cases (like invalid
topic alias used).

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>