Update imsg*.[ch] from OpenBSD, add some compat bits it needs and remove some

bits it doesn't.

compat/explicit_bzero.c

@@ -0,0 +1,15 @@
+/*	$OpenBSD: explicit_bzero.c,v 1.4 2015/08/31 02:53:57 guenther Exp $ */
+/*
+ * Public domain.
+ * Written by Matthew Dempsky.
+ */
+
+#include <string.h>
+
+#include "compat.h"
+
+void
+explicit_bzero(void *buf, size_t len)
+{
+	memset(buf, 0, len);
+}