Threads: task pointer stored in ngx_file_t.

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.