Add iflag=fullblock to dd

Normally, count=n means read(2) will be called n times on the input to dd. If
the read() returns short, as may happen when reading from a pipe, fewer bytes
will be copied from the input. With conv=sync the buffer is padded with zeros
to fill the rest of the block.

iflag=fullblock causes dd to continue reading until the block is full, so that
count=n means n full blocks are copied. This flag is compatible with illumos
and GNU dd and is used in the ZFS test suite.

Submitted by: Ryan Moeller
Reviewed by: manpages, mmacy@
MFC after: 1 week
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D21441