Based on patch by Pavel Raiskup.
Use SEEK_HOLE/SEEK_DATA feature of lseek on systems that support
it. This can make archiving of sparse files much faster.
Implement the --hole-detection option to allow users to select
hole-detection method.
* src/common.h (hole_detection_method): New enum.
(hole_detection): New global.
* src/sparse.c (sparse_scan_file_wholesparse): New function as a
method for detecting sparse files without any data.
(sparse_scan_file_raw): Rename from sparse_scan_file; with edits.
(sparse_scan_file_seek): New function.
(sparse_scan_file): Reimplement function.
* src/tar.c: New option --hole-detection
* tests/checkseekhole.c: New file.
* tests/.gitignore: Mention two test binaries.
* tests/Makefile.am: Add new tests.
* tests/testsuite.at (AT_SEEKHOLE_PREREQ): New macro.
Include sparse06.at.
* tests/sparse06.at: New test case.
* tests/sparse02.at: Force raw hole-detection method.
* tests/sparsemv.at: Likewise.
* tests/sparsemvp.at: Likewise.
* doc/tar.1: Document --hole-detection option.
* doc/tar.texi: Document hole-detection algorithms and
command-line options.
* NEWS: Document hole-detection.
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