XXX: the text of this commit will be rewritten. It will be based
on original text of d80a97def9a1db6f07f5d2e68f7ad6 with updates.
See https://reviews.freebsd.org/D44151 for year old discussions.
This version has two differencies compared to what had been checked in
and reverted back in spring 2024:
- Sendfile support, pretty straightforward
- The 34740937f7a4 made very restrictive checks on an empty STAILQ. Although my code had been using "invalid" empty STAILQ safely, the queue.h would assert on them. I decided to comply with new standards and hope compiler will optimize everything out.
Several next commits will be squashed into this commit. They are
provided as separate revisions to separate completely new unreviewed
code from code that had already been reviewed in D44151.