dcfldd

In the event that the input is from a stream which has a known size, it
would be nice to provide a method to provide the size to dcfldd for time
estimation purposes. This patch implements this as an extension to the
sizeprobe= option.

In the event that sizeprobe=XXX is provided and XXX is neither if or of, it
attempts to parse the value as a number in the same method as bs, etc
(allowing suffixes).

A situation where this would be useful is when a split file is going to be
restored. The size can be provided (either precisely or by estimate):

dcfldd if=/dev/sda sizeprobe=if split=1G of=test
cat test* | dcfldd of=/dev/sdb sizeprobe=100G

where sda is 100G and sdb is not (else, it would be possible to use
sizeprobe=of).

Dave