Fatalize all warnings in Perl code.

Fatalize all warnings in Perl code.

Search-and-replace change ‘use warnings;’ to ‘use warnings FATAL => 'all';’
in all Perl code.

Notwithstanding the dire cautions in ‘perldoc warnings’ about this,
I think it’s the right call for us.  One file was already doing it.
No new testsuite failures are observed on Linux with Perl 5.30.3
nor on NetBSD with Perl 5.6.1.

* bin/autoheader.in, bin/autom4te.in, bin/autoreconf.in
* bin/autoscan.in, bin/autoupdate.in, bin/ifnames.in
* lib/Autom4te/C4che.pm, lib/Autom4te/ChannelDefs.pm
* lib/Autom4te/Channels.pm, lib/Autom4te/Configure_ac.pm
* lib/Autom4te/FileUtils.pm, lib/Autom4te/General.pm
* lib/Autom4te/Request.pm, lib/Autom4te/XFile.pm:
  Make all warnings from the Perl interpreter into fatal errors.