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