Integrate perlio:

[ 17330]
PerlIO Layer implementation future proofing.
 - Inspired by Nick C's suggestion add size of function table to the table
   as a validation check.
 - also optimize pseudo-layer code to avoid malloc/link of something
   destined to be immediately popped & freed.
 - Minor addition to pod/perliol.pod
p4raw-link: @17330 on //depot/perlio: 2dc2558e5965ed21842375d8cc89de68e0c966e2

p4raw-id: //depot/perl@17331