MEDIUM: startup: load and parse configs from memory

MEDIUM: startup: load and parse configs from memory

Let's call load_cfg_in_ram() helper for each configuration file to load it's
content in some area in memory. Adapt readcfgfile() parser function
respectively. In order to limit changes in its scope we give as an argument a
cfgfile structure, already filled in init_args() and in load_cfg_in_ram() with
file metadata and content.

Parser function (readcfgfile()) uses now fgets_from_mem() instead of standard
fgets from libc implementations.

SPOE filter parses its own configuration file, pointed by 'config' keyword in
the configuration already loaded in memory. So, let's allocate and fill for
this a supplementary cfgfile structure, which is not referenced in cfg_cfgfiles
list. This structure and the memory with content of SPOE filter configuration
are freed immediately in parse_spoe_flt(), when readcfgfile() returns.

HAProxy OpenTracing filter also uses its own configuration file. So, let's
follow the same logic as we do for SPOE filter.