tar: port --overwrite symlink test to GNU/Hurd

tar: port --overwrite symlink test to GNU/Hurd

Problem reported by Pino Toscano in
<http://lists.gnu.org/archive/html/bug-tar/2012-01/msg00040.html>.
* gnulib.modules: Add fcntl-h, which defines
HAVE_WORKING_O_NOFOLLOW.
* src/extract.c (open_output_file): Use HAVE_WORKING_O_NOFOLLOW,
not O_NOFOLLOW, when testing whther O_NOFOLLOW works.

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