Integrate perlio:

[ 17258]
Last minute tinkering with PerlIO abstraction API.
 - PerlIO_importFILE and PerlIO_exportFILE now documented as taking
   const char *mode.
 - Other 'flags' field changed to U32
 - Discouraging words written about ":raw".
p4raw-link: @17258 on //depot/perlio: 4b069b44f3c785593c10e7aca80c893a2f210b9d

p4raw-id: //depot/perl@17259