tar: fix bug with -C and extracting directories

tar: fix bug with -C and extracting directories

Problem reported by Denis Excoffier in
<http://lists.gnu.org/archive/html/bug-tar/2010-10/msg00034.html>.

* src/extract.c (extract_dir): Use mkdirat, not mkdir.
* tests/extrac16.at: New file, to test for this bug.
* tests/Makefile.am (TESTSUITE_AT): Add it.
* tests/testsuite.at: Include it.

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