rename for_each_pidslot to for_each_child
rename pidslot vars to childno
more pidslot -> childno conversions

find_pid_slot -> find_childno
dump_pid_slots -> dump_childnos
PIDSLOT_NOT_FOUND -> CHILD_NOT_FOUND
EXIT_LOST_PIDSLOT -> EXIT_LOST_CHILD
slot -> childno variable replacements