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))
         {