ethtool: add LinkModeUpdate and Client.UpdateLinkMode

[dshynkev]

This proposes an interface for partial updates of link mode information. Tests coming up: wanted to make sure the overall design was acceptable first. The motivation behind disjoining LinkMode and LinkModeUpdate is that:

1. The sets of readable and writable fields are not the same, e.g. ETHTOOL_A_LINKMODES_MASTER_SLAVE_STATE is only readable and ETHTOOL_A_LINKMODES_LANES is only writable per docs.
2. This allows for a natural (for golang) partial update interface via nullable fields.

One point on which I waver is whether slices should have encoding/json-type semantics (nil <> empty slice) as proposed here, or perhaps be pointers like everything else. Let me know what you think.

[mdlayher]

LGTM, nice and tidy too. The genltest library is a bit awkward at times but I believe the existing tests should serve as a pretty good reference. Something I'd like to rework for netlink/genetlink v2, but such is life.

Thank you for contributing!

[mdlayher]

Note something like "Zero-value fields are not sent to the kernel as part of the update.", and that covers the nil and empty slice cases. I'm good with empty slices here; *[]AdvertisedLinkMode would be pretty awkward. I can't think of any valid reason you would try to clear all the advertised link modes via an empty slice anyway.

[SuperQ]

Ping, this needs a rebase.