Set up IPC to enable reloading the mappings file

My original plan was to use an `NSXPCConnection` to handle the IPC for
reloading mappings.

For some background, this is what I'm envisioning:

1. DomeKey is running in the background
2. User wants to make a change to their config. Update `mappings.dkmap`.
3. The new mappings file needs to be re-parsed and reloaded into memory.
   The simplest thing we could do is just quit and relaunch DomeKey. but
   that's kind of a pain. So instead, we run something like this from
   the command line:

      $ dome-key --reload-mappings

   This will tell the running DomeKey process to update its mappings by
   reloading the mappings file.

As I said, I was going to use `NSXPCConnection` for the IPC
communication, as it had come up in my research for IPC mechanisms on OS
X. I knew I didn't want to use a TCP socket as that seemed like too much
overhead. The ability to pass messages to classes "directly" using
`NSXPCConnection` was very appealing. However, I got a little lazy
trying to learn it. It has a whole model for how the communication
should work, including various procedures that need to be set up using
its API. It started to feel like a bit of a pain.

At that point I started looking into alternatives. One idea was to move
the IPC into the Rust code and use Unix Domain Sockets
(https://doc.rust-lang.org/std/os/unix/net/struct.UnixListener.html). I
was tempted to go this route, but I wasn't a fan of the need to always
be listening, the potential need to spawn a thread to reload the
mappings to free up the main workload, the need to figure out a way to
pass `State` (which keeps our parsed mappings in memory) around, and the
fact that only plain strings are passed, instead of messages in the case
of `NSXPCConnection`.

I came across
https://developer.apple.com/library/archive/documentation/MacOSX/Conceptual/OSX_Technology_Overview/SystemTechnology/SystemTechnology.html
where I discovered BSD notifications and `notify.3`. This seemed to be
the lightweight easy-to-use solution that I was looking for. After
following a little sample code, it works like a charm.

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