Use the rm command with -P as a shred fallback (#179)

* 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.

bin/_blackbox_common.sh

@@ -277,6 +277,9 @@ function shred_file() {
     #NOTE: srm by default uses 35-pass Gutmann algorithm
     CMD=srm
     OPT=-f
+  elif _F=$(mktemp); rm -P "${_F}" >/dev/null 2>/dev/null ; then
+    CMD=rm
+    OPT=-Pf
   else
     echo "shred_file: WARNING: No secure deletion utility (shred or srm) present; using insecure rm"
     CMD=rm