mac allocation policy prevents multiple links on rear ports

Originally filed as https://github.com/oxidecomputer/dendrite-archived/issues/669

We have a static policy for assigning MAC addresses to links on a switch. 32 MACs are reserved for rear ports and 128 for front ports. This lets us create up to 4 links on each QSFP/front port, but only a single link on each of the rear ports.

For production/customer installations, this assignment is fine. There is no reason to have more than a single link for each rear port in those scenarios. For diagnostic and/or bringup work, we may want to create up to one link for each lane on a rear port.

There are two fairly straightforward approaches to handling this. 1) we can add a "dynamic mac allocation" dpd flag, which would do dynamic mac allocation rather than static. 2) we can use the currently unassigned 95 MACs at the end of the block as an overflow pool.

No files, tests, or entry points are named in the issue. Start by tracing the static MAC assignment policy and its dpd configuration, then compare dynamic allocation with using the unassigned overflow pool. Done means diagnostic or bringup setups can create one link per lane on a rear port while production allocation remains unchanged.