Refactored sendfile() AIO preload. This reduces layering violation and simplifies the logic of AIO preread, since it's now triggered by the send chain function itself without falling back to the copy filter. The context of AIO operation is now stored per file buffer, which makes it possible to properly handle cases when multiple buffers come from different locations, each with its own configuration.

--- a/src/os/unix/ngx_files.h
+++ b/src/os/unix/ngx_files.h
@@ -375,6 +375,7 @@ size_t ngx_fs_bsize(u_char *name);
 
 #if (NGX_HAVE_FILE_AIO)
 
+ngx_int_t ngx_file_aio_init(ngx_file_t *file, ngx_pool_t *pool);
 ssize_t ngx_file_aio_read(ngx_file_t *file, u_char *buf, size_t size,
     off_t offset, ngx_pool_t *pool);