ip: Add a new family of commands, "stats"

ip: Add a new family of commands, "stats"

Add a core of a new frontend tool for interfacing with the RTM_*STATS
family of messages. The following patches will add subcommands for showing
and setting individual statistics suites.

Note that in this patch, "ip stats" is made to be an invalid command line.
This will be changed in later patches to default to "show" when that is
introduced.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David Ahern <dsahern@kernel.org>