zebra: Abstract the `netlink_socket()` API

zebra: Abstract the `netlink_socket()` API

The `netlink_socket()` function is used in many places to create and
initialize Netlink sockets. Currently, it can only create
`NETLINK_ROUTE` Netlink sockets.

This commit generalizes the behavior of the `netlink_socket()` function,
enabling it to generate Netlink sockets of any type. Specifically, it
extends the `netlink_socket()` function with a new argument `nl_family`,
which allows developers to specify the Netlink family of the socket to
be created.

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