Make share/stub.pl more robust

Changes:

    * Implement exec_tar() to execute one of gtar, tar or bsdtar in one of
      /usr/local/bin, /usr/bin or /bin, rather than executing tar(1) from
      $PATH

    * Remove usage of File::Temp::mkdtemp() in favor of a much more
      secure File::Temp::tempdir() with CLEANUP option