ieee802-eth-if: configurable auto-negotiation

[jovatn]

When a port is set to auto-negotiate, Infix always advertises all speed/duplex mode the port is capable of. In some situations, it is preferable to only advertise a (configurable) subset, e.g., 100-full or 1000-full.

A case of special interest is the use on SFP ports with dual/multi-speed fiber SFPs, where there is a need to configure the SFP to be run at a lower speed.

A sister issue: (a slightly related to configurable auto-neg is another feature request, "configurable flow-control" #501, but they can be done independently)