MEDIUM: config: make the frontends automatically bind to the listeners' processes

MEDIUM: config: make the frontends automatically bind to the listeners' processes

When a frontend does not have any bind-process directive, make it
automatically bind to the union of all of its listeners' processes
instead of binding to all processes. That will make it possible to
have the expected behaviour without having to explicitly specify a
bind-process directive.

Note that if the listeners are not bound to a specific process, the
default is still to bind to all processes.

This change could be backported to 1.5 as it simplifies process
management, and was planned to be done during the 1.5 development phase.