RFC: dd oflag=trunc to support in place filtering of files

[Pádraig Brady]

The thought just occurred to me that this could be useful
to filter large files in place? For example:

  grep whatever file.big | dd bs=1M conv=notrunc oflag=trunc

That would assume that grep never outputs more than it reads,
and would issue a final truncate along the lines of:

  ftruncate(STDOUT_FILENO, lseek(STDOUT_FILENO, 0, SEEK_CUR));

Useful enough to add?

Pádraig.