Merge of r4384, r4385:

Fixes for limit_rate:

*) Fixed throughput problems with large limit_rate.

   Previous attempt to fix this was in r1658 (0.6.18), though that one
   wasn't enough (it was a noop).

*) Fixed interaction of limit_rate and sendfile_max_chunk.

   It's possible that configured limit_rate will permit more bytes per
   single operation than sendfile_max_chunk.  To protect disk from
   takeover by a single client it is necessary to apply sendfile_max_chunk
   as a limit regardless of configured limit_rate.

   See here for report (in Russian):
   http://mailman.nginx.org/pipermail/nginx-ru/2010-March/032806.html