- Added a list_remove_first() which is generally better than list_remove()
provided you want to remove the first element.
- Added a list_append_list() to append and move all nodes from one list to
another.
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.