MINOR: dumpstats: split stats_dump_sv_stats() in two parts

This patch splits the function stats_dump_sv_stats() in two parts. The
extracted part is called stats_fill_sv_stats(), and just fill the stats buffer.
This split allows the usage of preformated stats in other parts of HAProxy
like the Lua.
diff --git a/include/proto/dumpstats.h b/include/proto/dumpstats.h
index 04f9e7e..a162e69 100644
--- a/include/proto/dumpstats.h
+++ b/include/proto/dumpstats.h
@@ -395,6 +395,8 @@
 int stats_fill_fe_stats(struct proxy *px, struct field *stats, int len);
 int stats_fill_li_stats(struct proxy *px, struct listener *l, int flags,
                         struct field *stats, int len);
+int stats_fill_sv_stats(struct proxy *px, struct server *sv, int flags,
+                        struct field *stats, int len);
 
 extern struct applet http_stats_applet;