Integrate perlio:

[  7711]
Minor tweaks:
consistent way of getting 'rb', 'wb' etc. for binary opens
move *perlio::layers to *open::layers
a #define to show layers available
DOSISH popen/PerlIO had export/import sense inverted.

p4raw-link: @7711 on //depot/perlio: a1d180c4a5147aa338f86b44d91b356bec0875ac

p4raw-id: //depot/perl@7713