dd: add 'fullblock' iflag

dd: add 'fullblock' iflag

Adds a fullblock iflag for improved compatibility with GNU dd.
The new iflag can be used to ensure that dd calls retrieve the
expected amount of data when reading from pipes or unusual
filesystems.

function                                             old     new   delta
packed_usage                                       32249   32334     +85
dd_main                                             1582    1632     +50
static.iflag_words                                    12      22     +10
------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 3/0 up/down: 145/0)             Total: 145 bytes

Signed-off-by: Nicholas Clark <nicholas.clark@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>