`prefsutil read` can now read a whole file

In addition to reading a specific preference with `prefsutil read <file> <key>` you can now read out an entire file's preferences in more-or-less human-readable form with `prefsutil read <file>`.

Note that this depends on the JSON parser to generate the human-readable output, and technically it's possible to have a plist that can't be JSON-encoded. But this probably won't come up, he said. A oppressive silence fell. Far off in the distance, a dog barked.

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