dd: new option: iflag=fullblock to accumulate full input blocks

dd: new option: iflag=fullblock to accumulate full input blocks

* src/dd.c (iread_fullblock): New function for reading full blocks.
(scanargs): Check for new parameter iflag=fullblock.
(skip): Use iread_fnc pointer instead of iread function.
(dd_copy): Use iread_fnc pointer instead of iread function.
* tests/dd/misc: Add test for dd - read full blocks.
* doc/coretuils.texi: Mention new parameter iflag=fullblock.
* NEWS: Mentioned the change.