Perlio internals re-organized.

Perlio internals re-organized.
 - open process creates AV of layer/arg pairs
   (appends layers from open() or open.pm to default list).
 - push arg is now an SV.
 - layer ->Open get passed the AV
 - open is no longer mandatory method.
 - topmost layer that has ->Open method does the open
 - any layers above are pushed once that returns.
 - vtable re-ordered so dummy layers need only provide push/pop methods.

p4raw-id: //depot/perlio@9314