netlink: add a basic socket diag parser of AF_NETLINK messages

netlink: add a basic socket diag parser of AF_NETLINK messages

* linux/netlink_diag.h (NDIAG_SHOW_GROUPS, NDIAG_SHOW_RING_CFG):
New macros.
* netlink_sock_diag.c: Include <linux/netlink_diag.h>,
"xlat/netlink_diag_show.h" and "xlat/netlink_states.h".
(decode_netlink_diag_req, decode_netlink_diag_msg): New functions.
(diag_decoders): Add AF_NETLINK.
* xlat/netlink_diag_show.in: New file.
* xlat/netlink_states.in: Likewise.

Co-authored-by: Fabien Siron <fabien.siron@epita.fr>