Add support for preserving ACL entries of files.

Add support for preserving ACL entries of files.

	* configure.ac (acl): New option.
	(HAVE_POSIX_ACL): Test for POSIX ACL support.  This is typically
	provided by libacl on GNU/Linux.

	* fileio.c (Ffile_acl, Fset_file_acl): New functions.
	(Fcopy_file): Change last arg to `preserve_extended_attributes'
	and copy ACL entries of file in addition to SELinux context if
	set.
	(syms_of_fileio): Add `file-acl' and `set-file-acl'.

	* Makefile.in (LIBACL_LIBS): New macro.
	(LIBES): Use it.

	* files.el (file-extended-attributes)
	(set-file-extended-attributes): New functions.
	(backup-buffer): Use them to handle both SELinux context and ACL
	entries.
	(backup-buffer-copy): Work with an alist of extended attributes,
	rather than an SELinux context.
	(basic-save-buffer-2): Ditto.

	* files.texi (File Attributes): Document ACL support and new
	`file-acl' function.
	(Changing Files): Mention argument name change of `copy-file' and
	document new function `set-file-acl'.