texi: warn against '.txi' and '.texinfo' input suffixes

texi: warn against '.txi' and '.texinfo' input suffixes

The warning being in the 'obsolete' category.  This is mostly to
ease transition to Automake-NG (see commit v1.12.1-416-gd5459b9),
and to discourage use of seldom-tested setups.

* automake.in (handle_texinfo_helper): Warn against Texinfo input
files with '.txi' or '.texinfo' suffixes.
* NEWS: Update.
* t/txinfo-other-suffixes.sh: Adjust and enhance.
* t/txinfo-no-repeated-targets.sh: No longer use '.txi' and
'.texinfo' extensions.

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