This hasn't actually been tested but, really, "how hard can this be?"

We're pretty good at avoiding race conditions (by moving stuff to
`dangerzone/processing`, mostly) but we definitely should avoid two
processors in parallel. Thankfully, it seems that systemd takes care
of this for us and will not run two services in parallel.

Obviously, the user can still shoot themselves in the foot and run
another processor in parallel, but that's when our race avoidance
stuff should save us.

Closes: #1