Scrub writes patterns on files or disk devices to make
retrieving the data more difficult. It operates in one of three
modes: 1) the special file corresponding to an entire disk is scrubbed
and all data on it is destroyed; 2) a regular file is scrubbed and
only the data in the file (and optionally its name in the directory
entry) is destroyed; or 3) a regular file is created, expanded until
the file system is full, then scrubbed as in 2).
