Fix the --no-overwrite-dir option

Fix the --no-overwrite-dir option

Given this option, tar failed to preserve permissions of empty directories
and to create files under directories owned by the current user that did
not have the S_IWUSR bit set.

* src/extract.c (fd_chmod): Rename to fd_i_chmod.
(fd_chmod): New function.
(safe_dir_mode): New function.
(extract_dir): Special handling for existing directories in
--no-overwrite-dir mode.
* tests/extrac23.at: New file.
* tests/Makefile.am: Add new test case.
* tests/testsuite.at: Likewise.