Work around a readdir bug in Darwin 7.9.0 (MacOS X 10.3.9) on HFS+

Work around a readdir bug in Darwin 7.9.0 (MacOS X 10.3.9) on HFS+
and NFS, whereby rm would not remove all files in a directory.
* src/remove.c (CONSECUTIVE_READDIR_UNLINK_THRESHOLD): Reduce to 10.
(NEED_REWIND): New macro, so that we incur the cost of the work-around
rewinddir only on afflicted systems.
* NEWS: Clarify and correct.
* tests/rm/readdir-bug: New file.  Test for the above fix.
* tests/rm/Makefile.am (TESTS): Add it.
Prompted by testing and analysis from Bruno Haible:
http://lists.gnu.org/archive/html/bug-coreutils/2006-09/msg00326.html