MEDIUM: config: authorize frontend and listen without bind.

MEDIUM: config: authorize frontend and listen without bind.

This allows to easily add/remove "bind" entries to a frontend without
being forced to remove it when the last entry is temporarily removed.
While "disabled" may sometimes work in a frontend, it becomes trickier
on "listen" sections which can also hold servers and be referenced by
other frontends.

Note that a "listen" section with no "bind" is equivalent to a "backend"
section.

Configs without any listeners are still reported as invalid and refuse
to load.