Use mkdtempat instead of mkdtemp

This fixes an interaction of -C with incremental 'X'.
Problem reported by Pavel Cahyna.
* gnulib.modules: Remove mkdtemp.
Add tempname, since our mkdtempat uses it.
* lib/mkdtempat.c, lib/mkdtempat.h: New files.
* lib/Makefile.am (noinst_HEADERS): Add mkdtempat.h.
(libtar_a_SOURCES): Add mkdtempat.c.
* src/incremen.c: Include mkdtempat.h.
(purge_directory): Use mkdtempat, not mkdtemp.

lib/Makefile.am

@@ -30,6 +30,7 @@ AM_CPPFLAGS = -I$(top_srcdir)/gnu -I../ -I../gnu
 AM_CFLAGS = $(GNULIB_WARN_CFLAGS) $(WERROR_CFLAGS)
 
 noinst_HEADERS = \
+ mkdtempat.h\
  paxlib.h\
  rmt.h\
  system.h\
@@ -37,6 +38,7 @@ noinst_HEADERS = \
  xattr-at.h
 
 libtar_a_SOURCES = \
+  mkdtempat.c\
   paxerror.c paxexit-status.c paxlib.h paxnames.c \
   rtapelib.c \
   rmt.h \

lib/mkdtempat.c

@@ -0,0 +1,45 @@
+/* Copyright 2026 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify it
+   under the terms of the GNU General Public License as published by the
+   Free Software Foundation; either version 3 of the License, or (at your
+   option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License along
+   with this program. If not, see <http://www.gnu.org/licenses/>.
+
+   Written by Paul Eggert.  */
+
+#include <config.h>
+
+#include "mkdtempat.h"
+
+#include <tempname.h>
+
+#include <stddef.h>
+#include <sys/stat.h>
+
+static int
+try_dir (char *tmpl, void *flags)
+{
+  int *pdirfd = flags;
+  return mkdirat (*pdirfd, tmpl, S_IRUSR | S_IWUSR | S_IXUSR);
+}
+
+/* Relative to the directory DIRFD if XTEMPLATE is relative,
+   generate a unique temporary directory from XTEMPLATE.
+   The last six characters of XTEMPLATE must be "XXXXXX";
+   replace them with a string that makes the generated directory unique.
+   Create the directory mode 700, and return its name.
+   On failure, return NULL and set errno.  */
+char *
+mkdtempat (int dirfd, char *xtemplate)
+{
+  return (try_tempname_len (xtemplate, 0, &dirfd, try_dir, 6) < 0
+	  ? NULL : xtemplate);
+}

lib/mkdtempat.h

@@ -0,0 +1 @@
+char *mkdtempat (int, char *);