A struct plugin_hub_internals was falsely casted to struct
plugin_callback_data. This caused the contained commands list pointer to point to
a struct hub_info and commands->size took the value of a pointer to a struct
net_connection. Since size is increased/decreased every time an item is
added to/removed from the list, this resulted in some funny crashes.
This fix is a little dirty as it exports some internals.
Assuming the argument definition "?xy", it was previously not possible
to specify only x. Also, the syntax will now be shown as "[x [y]]"
instead of "[x] [y]".
Tilka