qcopy-acl: Optimize copying of ACLs by directly copying the attributes. * lib/qcopy-acl.c (is_attr_permissions): New functions. (qcopy_acl): If USE_XATTR, copy the ACL related attributes directly. * m4/xattr.m4: New file. * modules/qcopy-acl (Files): Add it. (configure.ac): Invoke gl_FUNC_XATTR.