[New Port] sysutils/dwatch-json: FreeBSD dwatch profiles to produce JSON logs

DTrace module for FreeBSD dwatch(1) to produce JSON logs. JSON (JavaScript
Object Notation) is easily consumed by log aggregators such as Kafka and
Telegraf and visualizers like Vizceral. With these modules you can get
data out of the kernel/userland for use in such aggregators/visualizers.

Reviewed by: mat (previous version)
Approved by: mat (mentor)
Sponsored by: Smule, Inc.
Differential Revision: https://reviews.freebsd.org/D17316