MEDIUM: checks: Implement agent check using tcp-check rules

A shared tcp-check ruleset is now created to support agent checks. The following
sequence is used :

    tcp-check send "%[var(check.agent_string)] log-format
    tcp-check expect custom

The custom function to evaluate the expect rule does the same that it was done
to handle agent response when a custom check was used.
diff --git a/include/types/checks.h b/include/types/checks.h
index d3da5f2..d828331 100644
--- a/include/types/checks.h
+++ b/include/types/checks.h
@@ -180,8 +180,6 @@
 						 * rise to rise+fall-1 = good */
 	int rise, fall;				/* time in iterations */
 	int type;				/* Check type, one of PR_O2_*_CHK */
-	int send_string_len;			/* length of agent command string */
-	char *send_string;			/* optionally send a string when connecting to the agent */
 	struct server *server;			/* back-pointer to server */
 	struct proxy *proxy;                    /* proxy to be used */
 	char **argv;				/* the arguments to use if running a process-based check */
@@ -318,6 +316,7 @@
 #define TCPCHK_RULES_MYSQL_CHK   0x00000050
 #define TCPCHK_RULES_LDAP_CHK    0x00000060
 #define TCPCHK_RULES_SSL3_CHK    0x00000070
+#define TCPCHK_RULES_AGENT_CHK   0x00000080
 #define TCPCHK_RULES_SPOP_CHK    0x00000090
 
 /* A list of tcp-check vars, to be registered before executing a ruleset */