Import jsonwrt code from util-linux

Import jsonwrt code from util-linux

We've had a longstanding need to emit JSON to be friendlier
to shell scripting tools.

I hesitated for (way too long) on which JSON libraries to
use, but actually since we don't need to *parse* JSON,
we only need to omit it, the problem domain is super
simple.

util-linux has simple code for this
https://github.com/util-linux/util-linux/commit/b649ff92d2d91864e85a5bba4c89d52236817d9e

So this commit imports it, but it will need a little massaging
to build.

Signed-off-by: Colin Walters <walters@verbum.org>