[perl #57512] Warnings for implicitly closed handles

If the implicit close() fails, warn about it, mentioning $! in the
message.  This is a default warning in the io category.

When compiling with -Accflags=-DPERL_NOISY_IO_DESTROY the warning
‘Handle implicitly closed’ applies to any handle that is successfully
implicitly closed.  Enabling this warning in general provides obnox-
iously verbose output, since this practice is so widespread.  I
thought leaving it in might be useful as a tool to make the core Perl
code more robust against I/O errors.  It probably should not be docu-
mented, as such a tool for general use is better implemented as in a
CPAN module that overrides IO::Handle::DESTROY.

This currently produces warnings for code like this:

my $pid = open my $fh, "| perl -e0";
waitpid $pid, 0;
close $fh or die $!;
__END__

Output:
No child processes at - line 3.

I’m not so sure that should warn.  I am not sufficiently versed in
I/O to know when we do and do not want to warn, so I cannot proceed
much further with this.