libbpf: add the capability to specify netlink proto in libbpf_netlink_send_recv

This is a preliminary patch in order to introduce netlink_generic
protocol support to libbpf.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/7878a54667e74afeec3ee519999c044bd514b44c.1675245258.git.lorenzo@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>