backupfile, chdir-long, fts, savedir: make safer

backupfile, chdir-long, fts, savedir: make safer

* lib/backupfile.c (includes): Use "dirent--.h", since
numbered_backup can write to stderr during readdir.
* lib/savedir.c (includes): Likewise.
* lib/chdir-long.c (includes): Use "fcntl--.h", since openat
emulation can write to stderr on failure.
* lib/fts.c (includes) [!_LIBC]: Likewise for opendir and openat.
* lib/getcwd.c: Document why opendir_safer is unused.
* lib/glob.c: Likewise.
* lib/scandir.c: Likewise.
* lib/openat-proc.c: Likewise, for open_safer.
* modules/backupfile (Depends-on): Add dirent-safer.
* modules/savedir (Depends-on): Likewise.
* modules/fts (Depends-on): Add dirent-safer and openat-safer.
* modules/chdir-long (Depends-on): Add openat-safer.

Signed-off-by: Eric Blake <ebb9@byu.net>