* Use the `rm` command with `-P` as a shred fallback
The newer versions of OSX (Sierra) have neither `shred` nor `srm`.
They do have `rm` with the `-P` option, so we can fall back to that
before resorting to plain old `rm`.
* Add double-quotes and braces to the shell variable.
We should treat the variable as a string, and we should also be safe
when interpolating it.