Infrastructure to allow:

Infrastructure to allow:
  open($fh,"|-",@array);
to be implemented
i.e. mark pp_open as needing a stack mark, and make pp_open
process its args in that style (and pass them _all_ to tied handles OPEN).
Invent do_openn() which takes SV ** at allow it to see multiple args.
Note this does not _do_ anything yet.

p4raw-id: //depot/perlio@8484