hurd: Return EIO on non-responding interrupted servers

since we do not actually know whether the RPC was completed or not,
which makes a huge difference for e.g. write(), so better really error
out than letting caller think that the RPC did not happen.

	* hurd/intr-msg.c (_hurd_intr_rpc_mach_msg): When the server does not
	answer to interrupt_operation, return EIO instead of EINTR.