copy: detect smaller holes than the copy buffer size

Previously cp would not detect runs of NULs that were
smaller than the buffer size used for I/O (currently 128KiB).

* src/copy.c (copy_reg): Use an independent hole_size, set to
st_blksize, to increase the chances of detecting a representable hole,
in a run of NULs read from the input.
(create_hole): A new function refactored from sparse_copy() and
extent_copy() so we have a single place to handle holes.
(sparse_copy): Adjust to loop over the larger input buffer
in chunks of the passed hole size.  Also adjust to only call
lseek once per hole, rather than at least once per input buffer.
* tests/cp/sparse.sh: Add test cases for various sparse chunk sizes.
* NEWS: Mention the improvement.

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