integrate change#13453 from maint-5.6

integrate change#13453 from maint-5.6

       tweak change#11399 to use an explicit flag rather than
       ass_u_ming w32_pseudo_id will only be zero for the real
       toplevel interpreter; avoids multiple hosts from diddling
       the environment at the same time

p4raw-link: @13453 on //depot/maint-5.6/perl: 829f00ecc11a9cddae48275231aee0af40ec2be6
p4raw-link: @11399 on //depot/maint-5.6/perl: 0f7fbaf2aaedb3ed60637203319f491dc1812a50

p4raw-id: //depot/perl@13457
p4raw-integrated: from //depot/maint-5.6/perl@13456 'merge in'
	win32/perlhost.h (@12200..)