Code quality: Fix some cppcheck messages

This commit addresses these kind of errors:

portability: Passing NULL after the last typed argument to a variadic function leads to undefined behaviour. [varFuncNullUB]

Reason is that C does not guarantee that all pointer types are the same
size. See C99 6.2.5(27). cppcheck requires some sort of cast when NULL
is used as the last argument in a variadic list.

common/list.c

@@ -271,7 +271,7 @@ list_add_strdup_multi(struct list *self, ...)
     int rv = 1;
 
     va_start(ap, self);
-    while ((s = va_arg(ap, const char *)) != NULL)
+    while ((s = va_arg(ap, const char *)) != LIST_ADD_STRDUP_TERM)
     {
         if (!list_add_strdup(self, s))
         {

common/list.h

@@ -149,12 +149,16 @@ list_add_strdup(struct list *self, const char *str);
  *
  * This is a convenience function for a common operation
  * @param self List to append to
- * @param ... Strings to append. Terminate the list with a NULL.
+ * @param ... Strings to append. Terminate the list with LIST_ADD_STRDUP_TERM
  *
  * @result 0 if any memory allocation failure occurred. In this case
  * the list is unchanged.
  */
 
+/*
+ * We need a typed terminator to guarantee the stack object is the
+ * correct size (cf C99 std 6.2.5(12) for static checkers */
+#define LIST_ADD_STRDUP_TERM ((const char *)0)
 int
 list_add_strdup_multi(struct list *self, ...);