Maybe note that "some time now" refers to approximately 2003. :-)

I'm sure you know, but update this on the date you commit it.

This is an awkward sentence. I would suggest something like (ignoring mdoc formatting):

"random_fortuna is the only algorithm module available."

You are allowed to use the full 79-80 characters in manual page lines :-).

Ditto -- no need for the really short lines and they make it a bit harder to read.

Oh, I did not realize we would also be changing the API, but that makes sense.

Can this condition be shortened to just "optional random"? It seems like R || (R && !L) will always just be R.

(And/or, maybe it should instead be "optional random !random_loadable"?)

Ditto

Can we remove __packed? Given struct harvest_event is used in arrays, it seems like a bad idea to misalign most entries. Alternatively, he_bits could be replaced with an explicit padding byte.

This comment is now obsolete.

Is this print really useful? We don't print most harvest sources. Also consider that printf may go to system logs with a timestamp which may reduce the entropy from an attacker's perspective.

The Fortuna authors suggest only harvesting the low bits from timing events since the high bits are fairly predictable.

My understanding is that it doesn't really matter if we're well-seeded with a saved entropy file due to the properties of SHA_d-256, but if we *don't* have that and are still initially seeding we probably don't want to count highly-predictable high-order attach bytes (many of which are likely to be zero, and even the rest may be highly predictable from logs) towards our minimum seed size.

Maybe:

uint8_t attachentropy;

...

attachentropy = attachtime & 0xFF;
random_harvest_direct(&attachentropy, 1, RANDOM_ATTACH);

Trivial style(9) nit: the first /* should be on a line by itself.