Integrate perlio:

[  9332]
Fix un-init variable leading to spurious "Out of Memory!"

[  9331]
Tweak docs for C<open> (boy does that need wholesale revision...)
and update layers internals doc.

p4raw-link: @9332 on //depot/perlio: fe9745bfc279b016730696fd0a6abca4d493be60
p4raw-link: @9331 on //depot/perlio: b76cc8ba45957a82b545cf2a7b818233e6c0d507

p4raw-id: //depot/perl@9333