Cleanup this function to work with a symlinked directory. Document
it's behaviour more exactly, and add a simple unit-test for it (not
run by default, due to doing things like mounting and unmounting
system dirs on a live system, which doesn't seem safe for CI. But it
is useful for developers ensuring sanity).
Change-Id: I335316019ef948758392b03e91f9869102a472b9
