MINOR: proxy: implement unpublished backend keyword

MINOR: proxy: implement unpublished backend keyword

Add a new "unpublished" proxy keyword. This allows to start a backend
instance in unpublished state. This can be reverted via "publish
backend" on the CLI.

This keyword is restricted to backend and listen sections.

This patch is a simple feature implementation, however it can be
considered as a must-have when using dynamic backends. As such, it
should be backported up to 3.4.