dcb: Add a subtool for the DCB ETS object

dcb: Add a subtool for the DCB ETS object

ETS, for "Enhanced Transmission Selection", is a set of configurations that
permit configuration of mapping of priorities to traffic classes, traffic
selection algorithm to use per traffic class, bandwidth allocation, etc.

Add a dcb subtool to allow showing and tweaking of individual ETS
configuration options. For example:

    # dcb ets show dev eni1np1
    willing on ets_cap 8 cbs off
    tc-bw 0:0 1:0 2:0 3:0 4:100 5:0 6:0 7:0
    pg-bw 0:0 1:0 2:0 3:0 4:0 5:0 6:0 7:0
    tc-tsa 0:strict 1:strict 2:strict 3:strict 4:ets 5:strict 6:strict 7:strict
    prio-tc 0:1 1:3 2:5 3:0 4:0 5:0 6:0 7:0
    reco-tc-bw 0:0 1:0 2:0 3:0 4:0 5:0 6:0 7:0
    reco-tc-tsa 0:strict 1:strict 2:strict 3:strict 4:strict 5:strict 6:strict 7:strict
    reco-prio-tc 0:0 1:0 2:0 3:0 4:0 5:0 6:0 7:0

Signed-off-by: Petr Machata <me@pmachata.org>
Signed-off-by: David Ahern <dsahern@gmail.com>