ifstat: Add extended statistics to ifstat

ifstat: Add extended statistics to ifstat

Extended stats are part of the RTM_GETSTATS method. This patch adds them
to ifstat.
While extended stats can come in many forms, we support only the
rtnl_link_stats64 struct for them (which is the 64 bits version of struct
rtnl_link_stats).
We support stats in the main nesting level, or one lower.
The extension can be called by its name or any shorten of it. If there is
more than one matched, the first one will be picked.

To get the extended stats the flag -x <stats type> is used.

Signed-off-by: Nogah Frankel <nogahf@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>