tools: ynl-gen: switch to family struct

We'll want to store static info about the family soon.
Generate a struct. This changes creation from, e.g.:

	 ys = ynl_sock_create("netdev", &yerr);
to:
	 ys = ynl_sock_create(&ynl_netdev_family, &yerr);

on user's side.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>