MINOR: init: add HTTP client-only mode

MINOR: init: add HTTP client-only mode

Introduce the new <client_mode> global variable to define the operating mode
of haproxy. This variable can be set to 1 to allow haproxy to start without
any listeners.

During the initialization cycle, setting this flag 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.