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/proto/checks.h b/include/proto/checks.h
index 669c620..4dba34a 100644
--- a/include/proto/checks.h
+++ b/include/proto/checks.h
@@ -84,6 +84,8 @@
 int proxy_parse_spop_check_opt(char **args, int cur_arg, struct proxy *curpx, struct proxy *defpx,
 			       const char *file, int line);
 
+int set_srv_agent_send(struct server *srv, const char *send);
+
 #endif /* _PROTO_CHECKS_H */
 
 /*