Be able to get the security context even if configured with
--disable-acl, as security contexts are not ACLs, and the
main reason for --disable-acl was for efficiency with GNU ls -l,
a concern that does not apply to security contexts (which are
needed only with ls -Z). Problem reported by Pádraig Brady
<https://bugs.gnu.org/73418#52>.
* lib/acl.h (ACL_GET_SCONTEXT): New constant.
(aclinfo_free, aclinfo_scontext_free): Declare even if !USE_ACL.
* lib/file-has-acl.c (USE_LINUX_XATTR): No longer false merely
because !USE_ACL, because we need xattr to get scontext.
(get_aclinfo): Support new ACL_GET_SCONTEXT flag.