MINOR: tcpcheck_rule structure update

A new field is added into the tcpcheck_rule structure.
This field will host a string used as a comment to describe the rule.
Then this comment can be used in logs to report a more user friendly
message on the step which failed during the tcpcheck ruleset.
diff --git a/include/types/checks.h b/include/types/checks.h
index 3abebe1..252928f 100644
--- a/include/types/checks.h
+++ b/include/types/checks.h
@@ -214,6 +214,7 @@
 struct tcpcheck_rule {
 	struct list list;                       /* list linked to from the proxy */
 	int action;                             /* action: send or expect */
+	char *comment;				/* comment to be used in the logs and on the stats socket */
 	/* match type uses NON-NULL pointer from either string or expect_regex below */
 	/* sent string is string */
 	char *string;                           /* sent or expected string */