[feature] Provide a way to tell Perl not to close() a handle’s file descriptor

Per discussion just now with @Perl/perl-contributor

It’s a frustration point that, if you do:

open my $fh, '>>&=', $fd;

… Perl will close() the file descriptor once $fh is garbage-collected. While that’s useful behavior when $fd comes from, e.g., an exec() or file-descriptor pass via UNIX socket, if the $fd comes from an XSUB it’s often counterproductive since the XSUB may well want to use the file descriptor itself.

Currently this can be worked around by retaining a hash of ( $fd => $fh ), but “dangling filehandles” are rather ugly.

REQUEST: Provide a way to tell Perl that a given filehandle’s underlying file descriptor should not be close()d when the filehandle is garbage-collected.

Start with the issue discussion and the shown open() form to understand the requested file-descriptor lifetime behavior and the intended API. Define what should happen when the handle is garbage-collected, then add coverage demonstrating both preserved and normally closed descriptors; the issue is done when the behavior is documented and tested.