Copy filter: fixed sendfile aio handlers to set ctx->aio.

Sendfile handlers (aio preload and thread handler) are called within
ctx->output_filter() in ngx_output_chain(), and hence ctx->aio cannot
be set directly in ngx_output_chain().  Meanwhile, it must be set to
make sure loop within ngx_output_chain() will be properly terminated.

There are no known cases that trigger the problem, though in theory
something like aio + sub filter (something that needs body in memory,
and can also free some memory buffers) + sendfile can result in
"task already active" and "second aio post" alerts.

The fix is to set ctx->aio in ngx_http_copy_aio_sendfile_preload()
and ngx_http_copy_thread_handler().

For consistency, ctx->aio is no longer set explicitly in
ngx_output_chain_copy_buf(), as it's now done in
ngx_http_copy_thread_handler().