The computing of replacement bytes for overwriting undeletable files
uses random values and standard ones instead of the values provided in
option --bytes. Fix the mixup between both logics, mention the difference
in the documentation and reject defining both options --bytes and --undel.
When the new option --unused-fast is used, clusters which appear as wiped
are not written again. This is useful for avoiding virtual partitions to
be extended to their full size.
Contributed by michael
