Integrate maint branch io (pipe) fixes. (changes#16329,16330,16331)

Integrate maint branch io (pipe) fixes. (changes#16329,16330,16331)
p4raw-link: @16331 on //depot/maint-5.6/perl: 821302ef08e28e13ed846e63b71b1a8e0bd6e09a
p4raw-link: @16330 on //depot/maint-5.6/perl: 32b5515015764ee3e189442094a83142675fbf9b
p4raw-link: @16329 on //depot/maint-5.6/perl: 5676c1b12073c1e0625a9800fba3bbcb4a2e99c8

p4raw-id: //depot/perl@16528
p4raw-integrated: from //depot/maint-5.6/perl@16527 'merge in' pp_sys.c
	(@12026..)
p4raw-integrated: from //depot/maint-5.6/perl@16331 'merge in' util.c
	(@16329..)
p4raw-integrated: from //depot/maint-5.6/perl@16330 'merge in' doio.c
	(@14704..)