MEDIUM: proxy: implement publish/unpublish backend CLI

MEDIUM: proxy: implement publish/unpublish backend CLI

Define a new set of CLI commands publish/unpublish backend <be>. The
objective is to be able to change the status of a backend to
unpublished. Such a backend is considered ineligible to traffic : this
allows to skip use_backend rules which target it.

Note that contrary to disabled/stopped proxies, an unpublished backend
still has server checks running on it.

Internally, a new proxy flags PR_FL_BE_UNPUBLISHED is defined. CLI
commands handler "publish backend" and "unpublish backend" are executed
under thread isolation. This guarantees that the flag can safely be set
or remove in the CLI handlers, and read during content-switching
processing.