Close file descriptors on exec

Avoid FD leakage to children.

--- dhcp6_ctlclient.c.orig	Sun Jan 10 00:47:57 2016
+++ dhcp6_ctlclient.c	Sun Jan 10 00:48:32 2016
@@ -169,6 +169,7 @@ main(argc, argv)
 
 	s = -1;
 	for (res = res0; res != NULL; res = res->ai_next) {
+		res->ai_socktype |= SOCK_CLOEXEC;
 		s = socket(res->ai_family, res->ai_socktype,
 		    res->ai_protocol);
 		if (s < 0) {