tar: prefer openat-style functions

This change replaces traditional functions like 'open' with the
POSIX.1-2008 functions like 'openat'.  Mostly this is an internal
refactoring change, in preparation for further changes to close
some races.
* gnulib.modules: Add faccessat, linkat, mkfifoat, renameat, symlinkat.
Remove save-cwd.
* src/Makefile.am (tar_LDADD): Add $(LIB_EACCESS).
* tests/Makefile.am (LDADD): Likewise.
* src/common.h (chdir_fd): New extern var.
* src/compare.c (diff_file, diff_multivol): Use openat instead of open.
* src/create.c (create_archive, restore_parent_fd): Likewise.
* src/extract.c (create_placeholder_file): Likewise.
* src/names.c (collect_and_sort_names): Likewise.
* src/update.c (append_file): Likewise.
* src/compare.c (diff_symlink): Use readlinkat instead of readlink.
* src/compare.c (diff_file): Use chdir_fd instead of AT_FDCWD.
* src/create.c (subfile_open, dump_file0): Likewise.
* src/extract.c (fd_chmod, fd_chown, fd_stat, set_stat):
(repair_delayed_set_stat, apply_nonancestor_delayed_set_stat):
Likewise.
* src/extract.c (mark_after_links, file_newer_p, extract_dir):
(extract_link, apply_delayed_links):
Use fstatat rather than stat or lstat.
* src/misc.c (maybe_backup_file, deref_stat): Likewise.
* src/extract.c (make_directories): Use mkdirat rather than mkdir.
Use faccessat rather than access.  This fixes a minor permissions
bug when tar is running setuid (who would want to do that?!).
(open_output_file): Use openat rather than open.
In the process, this removes support for Masscomp's O_CTG files,
which aren't compatible with openat's signature.  Masscomp!  Wow!
That's a blast from the past.  As far as I know, that operating
system hasn't been supported for more than 20 years.
(extract_link, apply_delayed_links):
Use linkat rather than link.
(extract_symlink, apply_delayed_links):
Use symlinkat rather than symlink.
(extract_node): Use mknodat rather than mknod.
(extract_fifo): Use mkfifoat rather than mkfifo.
(apply_delayed_links): Use unlinkat rather than unlink or rmdir.
* src/misc.c (safer_rmdir, remove_any_file): Likewise.
* src/unlink.c (flush_deferred_unlinks): Likewise.
* src/extract.c (rename_directory): Use renameat rather than rename.
* src/misc.c (maybe_backup_file, undo_last_backup): Likewise.
* src/misc.c: Don't include <save-cwd.h>; no longer needed now
that we're using openat etc.
(struct wd): Add member fd.  Remove members err and fd.  All uses
changed.
(CHDIR_CACHE_SIZE): New constant.
(wdcache, wdcache_count, chdir_fd): New vars.
(chdir_do): Use openat rather than save_cwd.  Keep the cache up
to date.  This code won't scale well, but is good enough for now.
* src/update.c (update_archive): Use openat + fdopendir +
streamsavedir rather than savedir.

This file is a placeholder. It will be replaced with the actual ChangeLog
by make dist.  Run make ChangeLog if you wish to create it earlier.

src/unlink.c

@@ -77,7 +77,7 @@ flush_deferred_unlinks (bool force)
 	{
 	  if (p->is_dir)
 	    {
-	      if (rmdir (p->file_name) != 0)
+	      if (unlinkat (chdir_fd, p->file_name, AT_REMOVEDIR) != 0)
 		{
 		  switch (errno)
 		    {
@@ -101,7 +101,7 @@ flush_deferred_unlinks (bool force)
 	    }
 	  else
 	    {
-	      if (unlink (p->file_name) != 0 && errno != ENOENT)
+	      if (unlinkat (chdir_fd, p->file_name, 0) != 0 && errno != ENOENT)
 		unlink_error (p->file_name);
 	    }
 	  dunlink_reclaim (p);