Fixed sendfile in threads (or with aio preload) and subrequests. If sendfile in threads is used, it is possible that multiple subrequests will trigger multiple ngx_linux_sendfile_thread() calls, as operations are only serialized in output chain based on r->aio, that is, on subrequest level. This resulted in "task #N already active" alerts, in particular, when running proxy_store.t with "aio threads; sendfile on;". Fix is to tolerate duplicate calls, with an additional safety check that the file is the same as previously used. The same problem also affects "aio on; sendfile on;" on FreeBSD (previously known as "aio sendfile;"), where aio->preload_handler() could be called multiple times due to similar reasons, resulting in "second aio post" alerts. Fix is the same as well. It is also believed that similar problems can arise if a filter calls the next body filter multiple times for some reason. These are mostly theoretical though.

scgi_param  REQUEST_METHOD     $request_method;
scgi_param  REQUEST_URI        $request_uri;
scgi_param  QUERY_STRING       $query_string;
scgi_param  CONTENT_TYPE       $content_type;

scgi_param  DOCUMENT_URI       $document_uri;
scgi_param  DOCUMENT_ROOT      $document_root;
scgi_param  SCGI               1;
scgi_param  SERVER_PROTOCOL    $server_protocol;
scgi_param  REQUEST_SCHEME     $scheme;
scgi_param  HTTPS              $https if_not_empty;

scgi_param  REMOTE_ADDR        $remote_addr;
scgi_param  REMOTE_PORT        $remote_port;
scgi_param  SERVER_PORT        $server_port;
scgi_param  SERVER_NAME        $server_name;