Don't include dirname.h, since system.h does it now.

Don't include dirname.h, since system.h does it now.
(cache_fstatat, cache_stat_init): New functions.
(cache_statted, cache_stat_ok): New functions.
(write_protected_non_symlink): Remove struct stat ** buf_p arg,
which is no longer needed with the new functions.  All callers
changed.
(prompt, is_dir_lstat, remove_entry, remove_dir):
New struct stat * arg.  All callers changed.
(write_protected_non_symlink, prompt, is_dir_lstat, remove_entry):
(remove_cwd_entries, remove_dir, rm_1):
Use and maintain the file status cache.
(prompt, remove_entry): Omit the first "directory" in the diagnostic
"Cannot remove directory `foo': is a directory".  This causes "rm"
to pass a test case that it would otherwise fail now that it
"knows" more about its argument.  I think the diagnostic is better
without the first "directory" anyway.
(prompt): Remove the no-longer-needed IS_DIR arg; all callers changed.
(rm_1): Reject attempts to remove /, ./, or ../.