[InterNetNews/inn] bee870: Convert libinn(3) documentation to POD

[Julien ÉLIE]

  Branch: refs/heads/2.6
  Home:   https://github.com/InterNetNews/inn
  Commit: bee870b86f08d0bfa4eba201856519b90f4a742b
      https://github.com/InterNetNews/inn/commit/bee870b86f08d0bfa4eba201856519b90f4a742b
  Author: Julien ÉLIE <Julien-Elie at users.noreply.github.com>
  Date:   2022-02-09 (Wed, 09 Feb 2022)

  Changed paths:
    M MANIFEST
    M doc/man/Makefile
    R doc/man/libinn.3
    M doc/pod/Makefile
    A doc/pod/libinn.pod
    M include/inn/libinn.h

  Log Message:
  -----------
  Convert libinn(3) documentation to POD

This commits concludes a long series of roff man pages rewriting to POD.
All our roff man pages are now generated from POD.

Add subsections to better organize libinn(3) documentation.

Functions for setting or clearing file descriptor flags are now in a separate
header file (fdflag.h), and return a boolean.

Some arguments are also booleans, and not integers.