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.

Makefile

@@ -975,7 +975,7 @@ OBJS += src/mux_h2.o src/mux_fcgi.o src/mux_h1.o src/tcpcheck.o               \
         src/dynbuf.o src/wdt.o src/pipe.o src/init.o src/http_acl.o           \
         src/hpack-huff.o src/hpack-enc.o src/dict.o src/freq_ctr.o            \
         src/ebtree.o src/hash.o src/dgram.o src/version.o src/proto_rhttp.o   \
-        src/guid.o src/stats-html.o src/stats-json.o
+        src/guid.o src/stats-html.o src/stats-json.o src/stats-file.o
 
 ifneq ($(TRACE),)
   OBJS += src/calltrace.o