[MINOR] stats: use a dedicated state to output static data

It is a bit expensive and complex to use to call buffer_feed()
directly from the request parser, and there are risks that some
output messages are lost in case of buffer full. Since most of
these messages are static, let's have a state dedicated to print
these messages and store them in a specific area shared with the
stats in the session. This both reduces code size and risks of
losing output data.
diff --git a/include/types/session.h b/include/types/session.h
index 356f0ca..a8734ff 100644
--- a/include/types/session.h
+++ b/include/types/session.h
@@ -212,6 +212,9 @@
 			int ptr;		/* <0: headers, >=0 : text pointer to restart from */
 			int bol;		/* pointer to beginning of current line */
 		} errors;
+		struct {
+			const char *msg;	/* pointer to a persistent message to be returned in PRINT state */
+		} cli;
 	} data_ctx;				/* used by stats I/O handlers to dump the stats */
 	unsigned int uniq_id;			/* unique ID used for the traces */
 };