dcb: Add a subtool for the DCB PFC object

dcb: Add a subtool for the DCB PFC object

PFC, for "Priority-based Flow Control", allows configuration of priority
lossiness, and related toggles.

Add a dcb subtool to allow showing and tweaking of individual PFC
configuration options, and querying statistics. For example:

    # dcb pfc show dev eni1np1
    pfc-cap 8 macsec-bypass on delay 0
    pg-pfc 0:off 1:on 2:off 3:off 4:off 5:off 6:off 7:on
    requests 0:0 1:217 2:0 3:0 4:0 5:0 6:0 7:28
    indications 0:0 1:179 2:0 3:0 4:0 5:0 6:0 7:18

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