Devices may want to support more than one protocol, and for some devices
(e.g. Unifying peripherals stuck in bootloader mode) you might not even be able
to query for the correct protocol anyway.
The end year is legally and functionally redundant, and more importantly causes
cherry-pick conflicts when trying to maintain old branches. Use git for history.
That giant uint64_t isn't looking so big now, and we'll want to add even more
to it in the future. Split out some private flags that are never useful to the
client, although the #defines will have to remain until we break API again.
