rtnl_route: decode rtmsg netlink attributes

rtnl_route: decode rtmsg netlink attributes

* configure.ac (AC_CHECK_TYPES): Check for
rta_mfc_stats and rtvia structures in <linux/rtnetlink.h>.
* nlattr.c (decode_nla_ifindex): New function.
* nlattr.h (decode_nla_ifindex): New prototype.
* rtnl_route.c (decode_route_addr, decode_rta_metrics,
decode_rta_multipath, decode_rta_cacheinfo,
decode_rta_mfc_stats, decode_rtvia,
decode_rta_encap_type): New functions.
(rta_metrics_nla_decoders, rtmsg_nla_decoders): New arrays.
(decode_rtmsg): Use rtmsg_nla_decoders.
* xlat/lwtunnel_encap_types.in: New file.
* xlat/route_nexthop_flags.in: Likewise.
* xlat/rtnl_rta_metrics_attrs.in: Likewise.

Co-authored-by: Dmitry V. Levin <ldv@altlinux.org>