Integrate perlio:

[ 16335]
Fix the "PerlIO require leak". Snag was that clean_objs
autoloaded DESTROY (needing IO) after known layers had
been freed. Postpone layer list free to PerlIO_cleanup,
rather than PerlIO_destruct. Tweak sequence in perl_destruct
so that sv_undef is immortal till layer list is done with it.
p4raw-link: @16335 on //depot/perlio: 9f4bd2225c152364d8589911c4c769edae165d9b

p4raw-id: //depot/perl@16338