MINOR: init: add no listener mode

MINOR: init: add no listener mode

Introduce the new <no_listener_mode> global variable to define a new operating mode
for haproxy. This variable can be set to 1 to allow haproxy to start without
any listeners. Without such a setting, haproxy refuses to start without listener.

During the initialization cycle, setting this variable to 1 ensures that the
lack of configured listeners is no longer treated as a fatal error. This allows
programs based on haproxy source code to initialize the stack and use its
features even without a frontend. This will be the case for haload.