[InterNetNews/inn] 600100: Convert libinn(3) documentation to POD

[Julien ÉLIE]

  Branch: refs/heads/main
  Home:   https://github.com/InterNetNews/inn
  Commit: 60010006edc44b1114122105afced71b1b4e73a4
      https://github.com/InterNetNews/inn/commit/60010006edc44b1114122105afced71b1b4e73a4
  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.