Lexical use open ... support:

Lexical use open ... support:
  add ->cop_io to COP structure in cop.h.
  Make mg.c and gv.c associate it with ${^OPEN}.
  Make lib/open.pm set it.
  Have sv.c, perl.c, pp_ctl.c, op.c manipulate it in a manner
  manner similar to ->cop_warnings.
  Have doio.c's do_open9 and pp_sys.c's pp_backticks use it as default and
  call new PerlIO_apply_layers().
  Declare latter in perlio.h and define in perlio.c

p4raw-id: //depot/perlio@7740