Prepare for implementing extended attribute support.

This patch adds *xattr() stubs in case extended attribute support is
not present and implements the *attrat() family of functions.  Based
on the patch by Pavel Raiskup <pavel@raiskup.cz> for gnulib.

* acinclude.m4 (TAR_HEADERS_ATTR_XATTR_H): New defun.
* configure.ac: Call TAR_HEADERS_ATTR_XATTR_H
* lib/Makefile.am: Add xattr-at.[ch], distribute attr-xattr.in.h
[!TAR_COND_XATTR_H]: Build attr/xattr.h
* lib/attr-xattr.in.h: New file.
* lib/xattr-at.c: New file.
* lib/xattr-at.h: New file.

lib/attr-xattr.in.h

@@ -0,0 +1,58 @@
+/* Replacement <attr/xattr.h> for platforms that lack it.
+   Copyright (C) 2012 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/>.  */
+
+#ifndef TAR_ATTR_XATTR_H
+#define TAR_ATTR_XATTR_H
+#include <errno.h>
+
+/* setting */
+static inline int setxattr (const char *path, const char *name, const void
+                            *value, size_t size, int flags)
+{ errno = ENOTSUP; return -1; }
+
+static inline int lsetxattr (const char *path, const char *name, const void
+                             *value, size_t size, int flags)
+{ errno = ENOTSUP; return -1; }
+
+static inline int fsetxattr (int filedes, const char *name, const void *value,
+                             size_t size, int flags)
+{ errno = ENOTSUP; return -1; }
+
+
+/* getting */
+static inline ssize_t getxattr (const char *path, const char *name, void *value,
+                                size_t size)
+{ errno = ENOTSUP; return -1; }
+static inline ssize_t lgetxattr (const char *path, const char *name, void
+                                 *value, size_t size)
+{ errno = ENOTSUP; return -1; }
+static inline ssize_t fgetxattr (int filedes, const char *name, void *value,
+                                 size_t size)
+{ errno = ENOTSUP; return -1; }
+
+
+/* listing */
+static inline ssize_t listxattr (const char *path, char *list, size_t size)
+{ errno = ENOTSUP; return -1; }
+
+static inline ssize_t llistxattr (const char *path, char *list, size_t size)
+{ errno = ENOTSUP; return -1; }
+
+static inline ssize_t flistxattr (int filedes, char *list, size_t size)
+{ errno = ENOTSUP; return -1; }
+
+#endif
+