Use freezero(3) for the imsg framework in imsg_free(3) and ibuf_free(3).

In our privsep model, imsg is often used to transport sensitive
information between processes.  But a process might free an imsg, and
reuse the memory for a different thing.  iked uses some
explicit_bzero() to clean imsg-buffer but doing it in the library with
the freezero() is less error-prone and also benefits other daemons.
OK deraadt@ jsing@ claudio@

src/lib/libutil/imsg-buffer.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: imsg-buffer.c,v 1.9 2017/03/17 14:51:26 deraadt Exp $	*/
+/*	$OpenBSD: imsg-buffer.c,v 1.10 2017/04/11 09:57:19 reyk Exp $	*/
 
 /*
  * Copyright (c) 2003, 2004 Henning Brauer <henning@openbsd.org>
@@ -182,7 +182,7 @@ ibuf_free(struct ibuf *buf)
 {
 	if (buf == NULL)
 		return;
-	free(buf->buf);
+	freezero(buf->buf, buf->size);
 	free(buf);
 }
 

src/lib/libutil/imsg.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: imsg.c,v 1.14 2017/03/24 09:34:12 nicm Exp $	*/
+/*	$OpenBSD: imsg.c,v 1.15 2017/04/11 09:57:19 reyk Exp $	*/
 
 /*
  * Copyright (c) 2003, 2004 Henning Brauer <henning@openbsd.org>
@@ -263,7 +263,7 @@ imsg_close(struct imsgbuf *ibuf, struct ibuf *msg)
 void
 imsg_free(struct imsg *imsg)
 {
-	free(imsg->data);
+	freezero(imsg->data, imsg->hdr.len - IMSG_HEADER_SIZE);
 }
 
 int