Upstream: removed rudiments of upstream connection caching.

This functionality is now provided by ngx_http_upstream_keepalive_module.
Use NGX_FILE_ERROR for handling file operations errors.

On Win32 platforms 0 is used to indicate errors in file operations, so
comparing against either -1 or NGX_OK is not portable.

This was not much of an issue in patched code, since only ngx_fd_info() test
is actually reachable on Win32 and in worst case it might result in bogus
error log entry.

Patch by Piotr Sikora.