FastCGI: fixed start pointers in request buffers.

The start pointers are used in ngx_http_upstream_reinit() to
reinit FastCGI requests.
Upstream: fixed file buffers reinit in ngx_http_upstream_reinit().

Previously, a file buffer start position was reset to the file start.
Now it's reset to the previous file buffer end.  This fixes
reinitialization of requests having multiple successive parts of a
single file.  Such requests are generated by fastcgi module.