This addresses CVE-2025-45582.
* gnulib.modules: Add openat2.
* src/misc.c (open_subdir): New static function.
(fdbase_opendir): Use it.
* src/tar.c (open_searchdir_how): New var, replacing and
augmenting open_searchdir_flags. All uses changed.
* tests/extrac31.at: New file.
* tests/Makefile (TESTSUITE_AT), tests/testuite.at: Add it.