zebra: Add Generic Netlink socket

zebra: Add Generic Netlink socket

zebra already supports several Netlink sockets which allow it to
communicate with the kernel. Each Netlink socket has a specific purpose:
we have a socket for incoming events from the kernel, a socket for
programming the dataplane, a socket for the kernel messages, a socket
used as the command channel. All the currently supported sockets are
based on the `NETLINK_ROUTE` protocol.

This commit adds a new Netlink socket that allows zebra to send
commands to the kernel using the `Generic Netlink` protocol.

Signed-off-by: Carmine Scarpitta <carmine.scarpitta@uniroma2.it>