backupfile: new dir_fd args

backupfile: new dir_fd args

New module opendirat with code taken from fts.
Use this module to let backupfile use a directory file descriptor.
* NEWS: Document the incompatible change.
* lib/backup-find.c (find_backup_file_name):
* lib/backup-rename.c (backup_file_rename):
New arg DIR_FD.
* lib/backupfile.c: Include stdint.h, for SIZE_MAX.
(SIZE_MAX): Remove.
Include opendirat.h rather than dirent--.h.
(check_extension): New args DIR_FD and BASE_MAX.  All callers changed.
(numbered_backup): New args DIR_FD and PNEW_FD.  All callers changed.
(backupfile_internal): New arg DIR_FD.  All callers changed.
* lib/fts.c: Include opendirat.h.
(opendirat): Move to opendirat.c.
* lib/opendirat.c, lib/opendirat.h, modules/opendirat: New files.
* modules/backupfile (Depends-on): Remove dirfd, opendir.
Add opendirat.
* modules/fts (Depends-on): Remove fdopendir, openat-safer.
Add opendirat.