bulk uploads fail on multiple web servers

On architectures that use multiple web servers, such as Acquia Cloud or Pantheon, it cannot be assured that every request will be handled by the same server. If temporary files aren't stored in a location accessible from all servers, you'll get errors like:

The selected file 13967133KYYYH_fs.jpg could not be saved. The file is not in a known image format.

This patch defines a variable which can be used to specify a temporary directory used just for Image_fupload processing. Often you don't want to set the entire site to use such a directory as temp space because the frequency of writes there combined with the overhead of a distributed file system and spell performance troubles.

There is no UI for defining this variable - it would need to be set through drush or the $conf array. On Acquia Cloud a functioning value would look like:

drush vset image_fupload_tmp_dir '/var/www/html/[sitename]/docroot/sites/default/files/fuploadtmp