Stop external applications (dhclient) from catching signals.

There's a bug that prevents dhclient to be killed, so this patch
disables signal handling in external (non-bind9) applications.

This affects only ISC-DHCP.

lib/isc/unix/app.c

@@ -262,6 +262,9 @@ isc__app_ctxstart(isc_appctx_t *ctx0) {
 	ctx->want_reload = false;
 	ctx->blocked = false;
 
+	if (!isc_bind9)
+		return (ISC_R_SUCCESS);
+
 #ifndef HAVE_SIGWAIT
 	/*
 	 * Install do-nothing handlers for SIGINT and SIGTERM.