addons(ethtool): Add --set-channels option

[tomvil]

auto ens10f0
iface ens10f0 inet static
    channels-rx 0
    channels-tx 0
    channels-combined max

root@test:~# ethtool -l ens10f0
Channel parameters for ens10f0:
Pre-set maximums:
RX:		64
TX:		64
Other:		n/a
Combined:	64
Current hardware settings:
RX:		0
TX:		0
Other:		n/a
Combined:	64

[tomvil]

@julienfortin It has been a while 😄 will you merge this or should I close it?

[julienfortin]

Hi, you can leave it open. I haven't tested it yet but it looks okay. Overall i'm not a fan of if elif elif elif and the ways things are done today in the ethtool module, seems very slow to run all those commands all the time.
I'll make a debian upload before summer and will merge this before.

[sohorx]

Hi,

Interestingly enough, I did add set/get channels functions to the pyroute2 ethtool interface a few months ago: svinota/pyroute2#1192.

And, as I found this PR, I was wondering why ifupdown2 is not using pyroute2 in many cases. @julienfortin , is it intentional to avoid making pyroute2 a dependency ?