Integrate perlio:

[ 17478]
       Various core-dump preventions for cases Craig found on VMS.
       Fix PerlIO_exportFILE() to work with new PerlIOStdio_pushed.

[ 17479]
       (Re-)instate :pop as a "back door" to perl level layer stack
       manipulation.
p4raw-link: @17479 on //depot/perlio: 4ec2216f20a53a69267b31ce136e7410687cbe32
p4raw-link: @17478 on //depot/perlio: 81428673dc5737b28b793d38fc79696f8d6e80c4

p4raw-id: //depot/perl@17480