Patch to use pledge on OpenBSD.

Index: lib/isc/net.c
--- lib/isc/net.c.orig
+++ lib/isc/net.c
@@ -216,6 +216,13 @@ isc_net_probeunix(void) {
 
 static void
 try_ipv6only(void) {
+#ifdef __OpenBSD__
+/*
+ * pledge doesn't allow setting IPV6_V6ONLY, but that's ok,
+ * IPV6_V6ONLY is always enabled on OpenBSD;
+ */
+#undef IPV6_V6ONLY
+#endif
 #ifdef IPV6_V6ONLY
 	int s, on;
 #endif /* ifdef IPV6_V6ONLY */