MEDIUM: startup: move daemonization fork in init

MEDIUM: startup: move daemonization fork in init

Let's move daemonization fork in init(). We need to perform this fork always
before forking a worker process, in order to be able to launch master and then
its worker in daemon, i.e. background mode, if haproxy was started with '-D'
option.

This refactoring is a preparation step, needed for replacing then master-worker
fork in init() as well. This allows the master process not to read the whole
configuration file and not to do re-execution in order to free additional
memory, when worker was forked. In the new refactored design only the worker
process will read and apply a new configuration, while the master will arrive
very fast in its polling loop to wait worker's termination and to handle
signals. See more details in the following commits.