This is expected to ensure smoother operation on reload (and with less
chance of listen queue overflows).

Prodded by Igor Sysoev.

ZFS reports incorrect st_blocks until file settles on disk, and this
may take a while (i.e. just after creation of a file the st_blocks value
is incorrect).  As a workaround we now use st_blocks only if
st_blocks * 512 > st_size, this should fix ZFS problems while still
preserving accuracy for other filesystems.

The problem had appeared in r3900 (1.0.1).

Solaris has AT_FDCWD defined to unsigned value, and comparison of a file
descriptor with it causes warnings in modern versions of gcc.  Explicitly
cast AT_FDCWD to ngx_fd_t to resolve these warnings.

This inaccurate detection by using sysconf(_SC_NPROCESSORS_ONLN) can improve
usage of the mutex lock optimization on multicore systems.

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.