[InterNetNews/inn] 6bd0a5: Add support for LibreSSL

[Julien ÉLIE]

  Branch: refs/heads/main
  Home:   https://github.com/InterNetNews/inn
  Commit: 6bd0a51838b708ac9f3685789d70112f322cb455
      https://github.com/InterNetNews/inn/commit/6bd0a51838b708ac9f3685789d70112f322cb455
  Author: Julien ÉLIE <Julien-Elie at users.noreply.github.com>
  Date:   2021-12-05 (Sun, 05 Dec 2021)

  Changed paths:
    M doc/pod/hacking.pod
    M doc/pod/inn.conf.pod
    M doc/pod/install.pod
    M doc/pod/news.pod
    M nnrpd/misc.c
    M nnrpd/nnrpd.c
    M nnrpd/tls.c
    M readme.pod

  Log Message:
  -----------
  Add support for LibreSSL

A few systems like macOS and OpenBSD use LibreSSL by default.
Let's support it and integrate LibreSSL with INN.

We now handle properly OPENSSL_VERSION_NUMBER (which LibreSSL defines
as 2.0.0 -- a major OpenSSL version which never existed).  Some parts
of existing code was not appropriate for a build with LibreSSL.

And of course we also handle LIBRESSL_VERSION_NUMBER.

close #217