Merge of r4416:

Fixed AIO error handling on FreeBSD.

The aio_return() must be called regardless of the error returned by
aio_error().  Not calling it resulted in various problems up to segmentation
faults (as AIO events are level-triggered and were reported again and again).

Additionally, in "aio sendfile" case r->blocked was incremented in case of
error returned from ngx_file_aio_read(), thus causing request hangs.