MEDIUM: stats: remove the stats_sock struct from the global struct

MEDIUM: stats: remove the stats_sock struct from the global struct

Now the stats socket is allocated when the 'stats socket' line is parsed,
and assigned using the standard str2listener(). This has two effects :
  - more than one stats socket can now be declared
  - stats socket now support protocols other than UNIX

The next step is to remove the duplicate bind config parsing.