-freshen mode and -to option added

This new feature is again somehwat experimental.  With -freshen, all
data read is rewritten to disk.  It is done overlapping, such that each
sector is read and written two times.  The idea is to "freshen" the magnetic
data by overwriting it with itself.  Before you ask:  I needed this to
have some drives remap weakened (eroded) sectors which took ages to read.

The -to option is to give an end option for operation.  This way you can
give an end-position (positive value) or delta (number of bytes to operate
on with a negative value).  For example this will read a drive from
200MiB to 400MiB:

diskus -start 200M -to -200M -read /dev/sda

