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.