MEDIUM: stats: implement dump stats-file CLI

MEDIUM: stats: implement dump stats-file CLI

Define a new CLI command "dump stats-file" with its handler
cli_parse_dump_stat_file(). It will loop twice on proxies_list to dump
first frontend and then backend side. It reuses the common function
stats_dump_stat_to_buffer(), using STAT_F_BOUND to restrict on the
correct side.

A new module stats-file.c is added to regroup function specifics to
stats-file. It defines two main functions :
* stats_dump_file_header() to generate the list of column list prefixed
  by the line context, either "#fe" or "#be"
* stats_dump_fields_file() to generate each stat lines. Object without
  GUID are skipped. Each stat entry is separated by a comma.

For the moment, stats-file does not support statistics modules. As such,
stats_dump_*_line() functions are updated to prevent looping over stats
module on stats-file output.