MEDIUM: stats: reimplement HTTP keep-alive on the stats page

This basically reimplements commit f3221f9 ("MEDIUM: stats: add support
for HTTP keep-alive on the stats page") which was reverted by commit
51437d2 after Igor Chan reported a broken stats page caused by the bug
fix by previous commit.
diff --git a/include/proto/dumpstats.h b/include/proto/dumpstats.h
index 78ff6f7..61e39bf 100644
--- a/include/proto/dumpstats.h
+++ b/include/proto/dumpstats.h
@@ -31,6 +31,7 @@
 #define STAT_HIDE_DOWN  0x00000008	/* hide 'down' servers in the stats page */
 #define STAT_NO_REFRESH 0x00000010	/* do not automatically refresh the stats page */
 #define STAT_ADMIN      0x00000020	/* indicate a stats admin level */
+#define STAT_CHUNKED    0x00000040      /* use chunked encoding (HTTP/1.1) */
 #define STAT_BOUND      0x00800000	/* bound statistics to selected proxies/types/services */
 
 #define STATS_TYPE_FE  0