-c: pattern fill support.

The syntax is:

        -c hexc:nr[,hexc:nr,...]

As example, filling with 16 byte counter with following:

        ffffffff3333337a7a7a7a88882a0b0c

is possible with following pattern:

        -c ff:4,33:3,7a:4,88:2,2a:1,0b:1,0c:1

To fill with zeroes (or any other static byte) is simple:

        -c 0:16

Note that any bytes that tend to overflow counter space are dropped, and any
unused bytes not filled by pattern are set to zeroes.