uhub did not have this security bug since the hub did not advertise support for the
UCMD extension, but the message was still correctly relayed as specified in the
protocol specification.
However, this commit adds support for the UCMD extension, but only to the extent
that uhub will advertise it and uhub will also drop any such CMD message
generated by a client and will (currently) never issues a CMD message by itself.
This also deprecates the built-in chat_only configuration option.
If you need this functionality, then load the mod_chat_only plugin (if it is loaded then only operators
are able to search, connect, etc).
Dynamic commands are user commands that can be added dynamically to
the hub by a plugin.
The example plugin (mod_example.c) adds a !example command that when
invoked send a message to the user who invoked it.
