dd: add support for the conv=sparse option

Notes:
  Small seeks are not coalesced to larger ones,
  like is done in cache_round() for example.

  conv= is used rather then oflag= for FreeBSD compatibility.

* src/dd.c (final_op_was_seek): A new global boolean to flag
whether the final "write" was converted to a seek.
(usage): Describe the new conf=sparse option.
(iwrite): Convert a write of a NUL block to a seek if requested.
(do_copy): Initialize the output buffer to have a sentinel,
to allow for efficient testing for NUL output blocks.
If the last block in the file was converted to a seek,
then convert back to a write so the size is updated.
* NEWS: Mention the new feature.
* tests/dd/sparse: A new test for the feature.
* tests/Makefile.am: Reference the new test.

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