[InterNetNews/inn] 510b50: Fix reported low water marks on empty newsgroups

[Julien ÉLIE]

  Branch: refs/heads/2.7
  Home:   https://github.com/InterNetNews/inn
  Commit: 510b503de3277fd65de0312462ec02906ffa6d13
      https://github.com/InterNetNews/inn/commit/510b503de3277fd65de0312462ec02906ffa6d13
  Author: Julien ÉLIE <Julien-Elie at users.noreply.github.com>
  Date:   2023-01-21 (Sat, 21 Jan 2023)

  Changed paths:
    M doc/pod/news.pod
    M innd/ng.c
    M nnrpd/group.c
    M nnrpd/list.c
    M storage/buffindexed/buffindexed.c
    M storage/ovdb/ovdb.c
    M storage/ovsqlite/ovsqlite-server.c
    M storage/ovsqlite/sql-main.sql
    M storage/tradindexed/tdx-group.c

  Log Message:
  -----------
  Fix reported low water marks on empty newsgroups

The reported low water mark of empty newsgroups is one more than the reported
high water mark, whereas RFC 3977 recommends that the reported high water mark
be one less than the reported low water mark.

It notably fixes an overflow for an empty newsgroups with 2^31-1 as its
highest article number:

GROUP trigofacile.test.maxartnum
211 0 2147483648 2147483647 trigofacile.test.maxartnum

Also, the reported low/high water marks will be one less than what they
currently are.  It should be best for news clients as the reported low water
mark won't increase by 1 (when advertising an empty newsgroup) before the
arrival of the next article.

/!\ Do not change the active file as innd relies on the high water mark to
assign article numbers, so we shouldn't set it to one less than its expected
value.  Comment added to mention it.

Thanks to Bo Lindbergh for the clever patch related to ovsqlite.