MINOR: tools: add a new macro DEFVAL() to provide a default argument

This is like DEFZERO and DEFNULL, but this one allows to specify the
default value to be used as the first argument.

include/haproxy/tools-t.h

@@ -47,15 +47,20 @@
 /* return the largest possible integer of type <ret>, with all bits set */
 #define MAX_RANGE(ret) (~(typeof(ret))0)
 
+/* DEFVAL() returns either the second argument as-is, or <def> if absent. This
+ * is for use in macros arguments.
+ */
+#define DEFVAL(_def,...) _FIRST_ARG(NULL, ##__VA_ARGS__, (_def))
+
 /* DEFNULL() returns either the argument as-is, or NULL if absent. This is for
  * use in macros arguments.
  */
-#define DEFNULL(...) _FIRST_ARG(NULL, ##__VA_ARGS__, NULL)
+#define DEFNULL(...) DEFVAL(NULL, ##__VA_ARGS__)
 
 /* DEFZERO() returns either the argument as-is, or 0 if absent. This is for
  * use in macros arguments.
  */
-#define DEFZERO(...) _FIRST_ARG(NULL, ##__VA_ARGS__, 0)
+#define DEFZERO(...) DEFVAL(0, ##__VA_ARGS__)
 
 #define _FIRST_ARG(a, b, ...) b