Integrate perlio:

[  7740]
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-link: @7740 on //depot/perlio: ac27b0f573239284c298fcf96fb6c966551ef207

p4raw-id: //depot/perl@7742