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

A share tcp-check ruleset is now created to support SPOP checks. This way no
extra memory is used if several backends use a SPOP check.

The following sequence is used :

    tcp-check send-binary SPOP_REQ
    tcp-check expect custom min-recv 4

The spop request is the result of the function
spoe_prepare_healthcheck_request() and the expect rule relies on a custom
function calling spoe_handle_healthcheck_response().
diff --git a/include/types/checks.h b/include/types/checks.h
index a5676c1..d3da5f2 100644
--- a/include/types/checks.h
+++ b/include/types/checks.h
@@ -318,6 +318,7 @@
 #define TCPCHK_RULES_MYSQL_CHK   0x00000050
 #define TCPCHK_RULES_LDAP_CHK    0x00000060
 #define TCPCHK_RULES_SSL3_CHK    0x00000070
+#define TCPCHK_RULES_SPOP_CHK    0x00000090
 
 /* A list of tcp-check vars, to be registered before executing a ruleset */
 struct tcpcheck_var {