Use SEEK_HOLE for hole detection

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.

tests/testsuite.at

@@ -112,6 +112,19 @@ rm -f $[]$
 test $result -eq 0 || AT_SKIP_TEST
 ])
 
+dnl AT_SEEKHOLE_PREREQ
+m4_define([AT_SEEKHOLE_PREREQ],[
+checkseekhole || AT_SKIP_TEST
+])
+
+m4_define([AT_TIMEOUT_PREREQ],[
+timeout 100 true
+if test $? -ne 0; then
+    echo >&2 "the 'timeout' utility not found"
+    AT_SKIP_TEST
+fi
+])
+
 m4_define([AT_TAR_MKHIER],[
 install-sh -d $1 >/dev/null dnl
 m4_if([$2],,,&& genfile --file [$1]/[$2]) || AT_SKIP_TEST])
@@ -358,6 +371,7 @@ m4_include([sparse02.at])
 m4_include([sparse03.at])
 m4_include([sparse04.at])
 m4_include([sparse05.at])
+m4_include([sparse06.at])
 m4_include([sparsemv.at])
 m4_include([spmvp00.at])
 m4_include([spmvp01.at])