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

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

The following sequance is used :

    tcp-check send-binary "300C020101600702010304008000"

    tcp-check expect rbinary "^30" min-recv 14 \
        on-error "Not LDAPv3 protocol"

    tcp-check expect custom

The last expect rule relies on a custom function to check the LDAP server reply.
diff --git a/include/proto/checks.h b/include/proto/checks.h
index 322f9dd..ff43238 100644
--- a/include/proto/checks.h
+++ b/include/proto/checks.h
@@ -79,6 +79,8 @@
 				const char *file, int line);
 int proxy_parse_mysql_check_opt(char **args, int cur_arg, struct proxy *curpx, struct proxy *defpx,
 			    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);
 
 #endif /* _PROTO_CHECKS_H */