Integrate change #12711 from perlio;

Use ref count scheme rather than PerlLIO_dup() to do fp_dup().
Restores op/fork.t on Win32 (still segfault on exit of ok 2).

p4raw-link: @12711 on //depot/perlio: 93a8090dc33a049e3827420ced6c7db56ab1f529

p4raw-id: //depot/perl@12715

diff --cc perlio.c
Simple merge