rm-rf: refactor rm_rf_children(), split out body of directory iteration loop

This splits out rm_rf_children_inner() as body of the loop. We can use
that to implement rm_rf_child() for deleting one specific entry in a
directory.

(cherry picked from commit 1f0fb7d5)
(cherry picked from commit ca4a0e7d)
(cherry picked from commit 85ccac33)