The idea of a common handle is long revised and only misleading,
nl_handle really represents a socket with some additional
action handlers assigned to it.
Alias for nl_handle is kept for backwards compatibility.
This interface was internal so far which required all code defining
caches to be compiled with the sources available.
In order to simplify the interface, the co_msg_parser prototype was
changed to take the struct nl_parser_param directly instead of a
void *. It used to be void * because the co_msg_parser was directly
passed as the NL_CB_VALID callback function.
