dd: add count_bytes, skip_bytes and seek_bytes flags

dd: add count_bytes, skip_bytes and seek_bytes flags

dd now accepts the count_bytes and skip_bytes input flag and the
seek_bytes output flag, to more easily allow processing portions of a
file.

* src/dd.c (scanargs): Compute skip_records and skip_bytes when
'skip_bytes' iflag is used. Compute max_records and max_bytes when
'count_bytes' iflag is used. Compute seek_records and seek_bytes
when 'seek_bytes' oflag is used.
(skip_via_lseek): Use new 'bytes' parameter and handle potential
'records' equals to zero. Update the bytes parameter when called with
'fdesc' equal to STDOUT_FILENO. Update the header comments.
(dd_copy): Skip accordingly to skip_records AND skip_bytes. Count
accordingly to max_records AND max_bytes. Seek on output accordingly
to seek_records AND seek_bytes.
* NEWS (New features): Mention it.
* doc/coreutils.texi (dd invocation): Detail new flags and behaviors.
* tests/dd/bytes: New file. Tests for these new flags.
* tests/Makefile.am (TESTS): Add it.