dist: fix bugs #81558 and #81557.

From https://bugs.gnu.org/81558 and https://bugs.gnu.org/81557
(Bruno Haible).

The filter for overly long file names had as many dots as the
maximum, so it rejected names of exactly that length; it needs one
more.

99 characters is the v7 limit, not a portable limit for tarballs in
general; ustar, the default since 1.18, stores longer names.  The
manual said otherwise.

* bin/automake.in (handle_dist): one dot more than the maximum.
* lib/am/distdir.am (distdir-am): name the option in the diagnostic.
* doc/automake.texi (List of Automake options): rewrite the
filename-length-max entry; the limit depends on the archive format,
and the length counted includes the $(distdir)/ prefix.
* t/filename-length-max.sh: new test.
* t/list-of-tests.mk (handwritten_TESTS): add it.
* NEWS: mention both.