Add "temporary" option, and ability to create files.

Add "temporary" option, and ability to create files.

This commit:
* Adds a "temporary" option, which causes a unique file to be
  created, which is unliked as soon as it is created (and thus
  will not be present on exit). This is used for creation of
  temporary disks.
* Will create a file, if "filesize" is specified and the file
  is not present or is zero length (useful in conjunction with the
  above).