pool: display URL instead of "Pool N" if no poolname provided.

This works since get_pool_name() in pool.c checks if an empty string is
provided, and uses the URL instead if so.

sgminer.c

@@ -528,7 +528,7 @@ struct pool *add_pool(void)
 
 	/* Default pool name */
 	char buf[32];
-	sprintf(buf, "Pool %d", pool->pool_no);
+	sprintf(buf, "", pool->pool_no);
 	pool->poolname = strdup(buf);
 
 	pools = (struct pool **)realloc(pools, sizeof(struct pool *) * (total_pools + 2));