Snapshot of new PerlIO open scheme. Still buggy - mainly in open($fh,">&STDOUT!")

Snapshot of new PerlIO open scheme. Still buggy - mainly in open($fh,">&STDOUT!")
type code.
  - Invent PerlIO_openn() - which has "lots" of args a bit like do_openn() which
    is its main caller. In particular now has access to "extra" args, and
    can tell when an open handle is "reopened" (or duped?).
  - In -Duseperlio PerlIO_open() et. al. are now wrappers on PerlIO_openn().
  - In -Uuseperlio (untested as yet) PerlIO_openn() is a wrapper on
    PerlIO_open() et. al. (i.e. other way round).
  - Collapse "vtable" entries for layers - was fdopen/open/reopen now just open
    with args close to PerlIO_openn().

p4raw-id: //depot/perlio@9302