[MEDIUM] implement the CSV output for the statistics

It is now possible to get CSV ouput from the statistics by
simply appending ";csv" to the HTTP request sent to get the
stats. The fields keep the same ordering as in the HTML page,
and a field "pxname" has been prepended at the beginning of
the line.
diff --git a/include/types/session.h b/include/types/session.h
index a768619..347bbaa 100644
--- a/include/types/session.h
+++ b/include/types/session.h
@@ -74,8 +74,10 @@
 #define	SN_FINST_SHIFT	12		/* bit shift */
 /* unused:              0x00008000 */
 
+/* Note: those flags must move to another place */
 #define SN_STAT_HIDEDWN	0x00010000	/* hide 'down' servers in the stats page */
 #define SN_STAT_NORFRSH	0x00020000	/* do not automatically refresh the stats page */
+#define SN_STAT_FMTCSV	0x00040000	/* dump the stats in CSV format instead of HTML */
 
 
 /* WARNING: if new fields are added, they must be initialized in event_accept()