random(4): Abstract loader entropy injection

[cem]

Summary

Break random_harvestq_prime up into some logical subroutines.

No functional change.

The goal is that it becomes easier to add other early entropy sources.

[delphij]

I think the proposed change actually does change the code behavior slightly.

What the old code does is basically "if we don't see an object of type 'boot_entropy_cache', try looking up the one with type '/boot/entropy' instead".

The new code validates both whether the object of 'boot_entroy_cache' is preloaded (existing behavior), and if the loaded object was sane (newly added behavior; previously we would not check the sanity before proceeding further, and it seemed to be reasonable because the expectation was that the type is either boot_entropy_cache, or '/boot/entropy' and the loaded file would be the same one).

The compatibility code wrapped in the NO_BACKWARD_COMPATIBILITY block was delivered in 12.0-RELEASE, but we didn't issued a warning that "please please update your /boot/defaults/loader.conf now" in the fallback case (we should have done that ~3 years ago, but now it's probably less relevant).

I would suggest that we simply delete that compatibility shim instead: it wasn't the intention to support both types permanently, and the code is already short enough to fit in one page.

[cem]

Drop NO_BACKWARD_COMPATIBILITY shim.

[delphij]

Thanks!

[delphij]

(Note that the refactor is not really needed, but I don't have strong opinion here).