Integrate perlio:

[ 14214]
Win32-ize socketpair test
- Win32 can fork even though $Config{d_fork} is undef
- SOCK_DGRAM does not work - skip those tests.

[ 14213]
Abstract out the cloning of SvPVX and handle shared pv in a
safe (if suboptimal) manner. Does not fix op/fork.t :-(

p4raw-link: @14214 on //depot/perlio: 26bf1728b5f258bdc26021f9eb583c46488b8859
p4raw-link: @14213 on //depot/perlio: 83841fad1c8ce4928a4239052849bad556adb3d3

p4raw-id: //depot/perl@14218

diff --cc sv.c
Simple merge