/etc/rc.d/rc.shutdown : Reworked the ugly STRAYPARTandMNT

part by wrapping the old one into unused function retired_unmount_partitions
and wrote new with some code parts of the old code reused.
Since it calls the killzombies function, made it test for killable PID
by using /bin/ps --no-header -p ONEZOMBIE since it seems that fuser pid
fragments are around, but unkillable. Further use /bin/ps instead of
just ps if busybox is compiled to lookup internal applets first.