Close file descriptors on exec

Avoid FD leakage to children.

--- dhcp6_ctl.c.orig	Sun Jan 10 00:47:08 2016
+++ dhcp6_ctl.c	Sun Jan 10 00:47:47 2016
@@ -97,6 +97,7 @@ dhcp6_ctl_init(addr, port, max, sockp)
 		    gai_strerror(error));
 		return (-1);
 	}
+	res->ai_socktype |= SOCK_CLOEXEC;
 	ctlsock = socket(res->ai_family, res->ai_socktype, res->ai_protocol);
 	if (ctlsock < 0) {
 		dprintf(LOG_ERR, FNAME, "socket(control sock): %s",