Threads: task pointer stored in ngx_file_t. This simplifies the interface of the ngx_thread_read() function. Additionally, most of the thread operations now explicitly set file->thread_task, file->thread_handler and file->thread_ctx, to facilitate use of thread operations in other places. (Potential problems remain with sendfile in threads though - it uses file->thread_handler as set in ngx_output_chain(), and it should not be overwritten to an incompatible one.) In collaboration with Valentin Bartenev.