Introduces the functions genl_register_family() and
genl_unregister_family() to register a Generic Netlink family
which does not implement a cachable type.
API users can direct received messages into genl_handle_msg() which
will validate the messages and call the callback functions defined
in the commands definition.
See test/test-genl.c for an example on how to use it.
Dumping objects as environment variables has never been implemented
completely and only increases the size of the library for no real
purpose. Integration into scripts is better achieved by implementing
a python module anyway.