Build automation streamlined: use golden-image zones to start up builds

[jimklimov]

As reminded by a recent activity for #6824 and discussion in #6828, some time ago I wrote https://github.com/jimklimov/illumos-zonemgr which automates "discardable image" local zone creation from "golden image" zones, initially made with intent as builder zones but also could be used for streamlined infra (e.g. standard webserver setup, to clone software and paste content into, etc.). By design, it is a scriptware to manage an arbitrary zone that zoneadm supports, so not a new brand for limited use.

Due to various historic reasons (time and focus mostly) it got suspended close to finish, so IIRC it had a functional skeleton for lifecycle management of the zones, but did not yet have the integration with "pkg" to install the desired baseline of build requirements for a particular build (such as a list provided from oi-userland makefiles), nor a way to detect the best-suited golden image to clone (least modifications needed to tailor the clone to a build, and possibly that we only add packages and not remove anything preinstalled - following same logic as OBS does for preinstall archives).

That latter point, finding a best-suited golden image zone, could be used for maintaining a number of baselines for different goals (e.g. small generic footprint, with one specialized GIZ for C/C++ builds and others for Java/Python/Go/... package builds).

Discussion in #6828 suggested that the skeleton script, if it is as usable as I remember it, can help oi-userland builds already and the installation of packages can be externalized in a way that oi-userland build automation would provide and tailor for its needs, or contributed into the script to complete fleshing the skeleton :)