FR: Output format JSON

[eopo]

Hi,

to be able to process the decoded telegrams more easily and less error-prone, output as JSON would be very handy.
I would therefore like to have an output format flag, that takes in "JSON" as option.

Keep up the great work in this really valuable project.

[lingfish]

For anyone interested... I've started work on this.

First and foremost -- it's been many, many years since I coded in C, some of this might be a little rough!

I can also see more "optimised" ways of doing this, but it would mean a pretty major refactor that I'm not keen on doing.

Points of note:

1. It will (eventually) output either the decode, with "demod_name" being the decoded type, or
2. A JSON struct in the form of {"error": "whatever the error was"} plus keeping the existing error output to file descriptor

A normal struct would look like this:

{
    "demod_name": "POCSAG512",
    "address": 164642,
    "function": 0,
    "alpha": "HbSOME PAGER MESSAGE<NUL><NUL>"
}

An error:

{
    "error": "whatever the error was"
}

It will output in JSON newline-delimited style.

My working branch is here:

https://github.com/lingfish/multimon-ng/tree/add-json