We don't need the follow the pattern of checking wc -l against
/proc/mounts is 0, we care about any match, so we can just use
grep and it's exit code inside the if.
Change-Id: Idd0d78a613f8b43f169a320527c763d0bff74368
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
