[all] use explicit void parameter lists

This makes it a compile-time error to call a function that semantically
takes no parameters with a nonzero number of arguments.

Previously, such code would still compile, but risk blowing up the stack
if a compiler chose to use something other than caller-cleanup calling
conventions.

client/src/ll.c

@@ -38,7 +38,7 @@ struct ll
   LG_Lock lock;
 };
 
-struct ll * ll_new()
+struct ll * ll_new(void)
 {
   struct ll * list = malloc(sizeof(struct ll));
   list->head  = NULL;