Integrate perlio:

[  7736]
Basic tweaks to do_open() type parsing to allow layer/discipline
part to be isolated.

[  7727]
Experiment on use of attributes.pm interface.
Valid generic fix to auto-vivify code in rv2gv - only "upgrade" to
SVt_PVRV if not already something better (else vivify of say magic gets
core dump).

p4raw-link: @7736 on //depot/perlio: b931b1d952313afa398828ff4b2a40af20cfa65a
p4raw-link: @7727 on //depot/perlio: b13b21351ea0b61f5fdc4e4ab614bbe813c655f7

p4raw-id: //depot/perl@7739