MEDIUM: checks: Add a list of vars to set before executing a tpc-check ruleset

A list of variables is now associated to each tcp-check ruleset. It is more a
less a list of set-var expressions. This list may be filled during the
configuration parsing. The listed variables will then be set during each
execution of the tcp-check healthcheck, at the begining, before execution of the
the first tcp-check rule.

This patch is mandatory to convert all protocol checks to tcp-checks. It is a
way to customize shared tcp-check rulesets.
diff --git a/include/proto/checks.h b/include/proto/checks.h
index fb33746..d0c827b 100644
--- a/include/proto/checks.h
+++ b/include/proto/checks.h
@@ -63,6 +63,8 @@
 }
 
 void deinit_proxy_tcpcheck(struct proxy *px);
+int dup_tcpcheck_vars(struct list *dst, struct list *src);
+
 /* Declared here, but the definitions are in flt_spoe.c */
 int spoe_prepare_healthcheck_request(char **req, int *len);
 int spoe_handle_healthcheck_response(char *frame, size_t size, char *err, int errlen);