rm -r: avoid O(n^2) performance for a directory with very many entries

rm -r: avoid O(n^2) performance for a directory with very many entries

This enhancement works around a problem that is specific to at least
ext3 and ext4 file systems.  With them, it would take hours to remove
a two-million-entry directory.  RAM-backed file systems (tmpfs) are
not affected, since there is no seek penalty.
* remove.c (rm_malloc, rm_free, compare_ino): New functions.
(dirent_count, preprocess_dir): New function.
[struct readdir_data]: New struct.
(remove_cwd_entries): Call preprocess_dir.
* tests/rm/ext3-perf: New file.  Test for the performance fix.
* NEWS: mention the new feature