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/proto/checks.h b/include/proto/checks.h
index ff43238..669c620 100644
--- a/include/proto/checks.h
+++ b/include/proto/checks.h
@@ -81,6 +81,8 @@
 			    const char *file, int line);
 int proxy_parse_ldap_check_opt(char **args, int cur_arg, struct proxy *curpx, struct proxy *defpx,
 			       const char *file, int line);
+int proxy_parse_spop_check_opt(char **args, int cur_arg, struct proxy *curpx, struct proxy *defpx,
+			       const char *file, int line);
 
 #endif /* _PROTO_CHECKS_H */