Merge of r4920, r4939: ngx_write_chain_to_file() fixes.

*) Core: added debug logging of writev() in ngx_write_chain_to_file().

*) Core: fixed ngx_write_chain_to_file() with IOV_MAX reached.

   Catched by dav_chunked.t on Solaris.  In released versions this might
   potentially result in corruption of complex protocol responses if they
   were written to disk and there were more distinct buffers than IOV_MAX
   in a single write.