Multiple subdirectories are not automatically created for you

This is a bug that affects Drupal 6 only. If you set up a file component with a path of "files/webform/sub1/sub2" and sub1 does not exist, you get an error that the directory sub2 could not be created. This is because in Drupal 6, directories are not created recursively. This patch adds a simple mkdir() that uses the recursive option. It also has the nice side-effect of silencing the message about a new directory being created.