texi: warn against suffix-less info files

texi: warn against suffix-less info files

The warning being in the 'obsolete' category.  This is mostly to
ease transition to Automake-NG (see commit v1.12.1-392-ga0c7b6a),
and to discourage use of seldom-tested setups.

* automake.in (scan_texinfo_file): Warn against '@setfilename'
directives that specify suffix-less output info files.
* t/txinfo-without-info-suffix.sh: Adjust and enhance.
* t/txinfo-makeinfo-error-no-clobber.sh: No longer use suffix-less
info files in '@setfilename' directives.
* t/primary-prefix-valid-couples.sh: Likewise.
* t/txinfo-setfilename-repeated.sh: Likewise.
* t/txinfo-vtexi2.sh : Likewise.
* t/mdate2.sh: Likewise.
* NEWS: Update.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>