MINOR: init: allow a fileless init mode

MINOR: init: allow a fileless init mode

This patch provides the possibility to initialize haproxy without
configuration file. This may be identified by the new global and exported
<fileless_mode> and <fileless_cfg> variables which may be used to
provide a struct cfgfile to haproxy by others means than a physical
file (built in memory).
When enabled, this fileless mode skips all the configuration files
parsing.