lib: xpc: implement support for iterating through directories

lib/libfs/include/fs/file.h

@@ -19,6 +19,8 @@ struct fs_file_ops {
 		size_t,
 		off_t *);
 	enum fs_status (*f_seek)(struct fs_file *, off_t, int);
+	enum fs_status (
+		*f_readdir)(struct fs_file *, struct xpc_buffer *, off_t *);
 };
 
 extern struct fs_inode *fs_file_get_inode(const struct fs_file *f);
@@ -37,4 +39,8 @@ extern enum fs_status fs_file_write(
 	const struct xpc_buffer *buf,
 	size_t count);
 
+extern enum fs_status fs_file_readdir(
+	struct fs_file *f,
+	struct xpc_buffer *buf);
+
 #endif