file unmanaged save upload
I suggest using file_save_upload() anyway, beacause (as the API docs say):


  The file will be added to the {file_managed} table as a temporary file. Temporary files are periodically cleaned. To make the file a permanent file, assign the status and use file_save() to save the changes.


So you don't need to worry about having the file "managed" by Drupal as long as you don't change the status of the file to FILE_STATUS_PERMANENT.

Otherwise, if you really don't want this default behaviour (and also no hooks to get invoked), just copy the file in your form_submit() using file_unmanaged_copy() (give it uploaded tmp file as source).