Upgrade unbound library

These files were pulled from the 1.6.3 release tarball.

This new version builds against OpenSSL version 1.1 which will be
the default in the new Debian Stable which is due to be released
RealSoonNow (tm).

external/unbound/compat/explicit_bzero.c

@@ -6,7 +6,11 @@
 #include "config.h"
 #include <string.h>
 
+#ifdef HAVE_ATTR_WEAK
 __attribute__((weak)) void
+#else
+void
+#endif
 __explicit_bzero_hook(void *ATTR_UNUSED(buf), size_t ATTR_UNUSED(len))
 {
 }