Move slaacd to /sbin

jca points out that all the other interface configuration tools live
there (like ifconfig or dhclient). Furthermore it starts so early in
the boot process that /usr might not be mounted yet if it's a nfs
filesystem.
sthen and deraadt agree

src/etc/rc.d/slaacd

@@ -1,8 +1,8 @@
 #!/bin/sh
 #
-# $OpenBSD: slaacd,v 1.1 2017/05/29 10:24:06 florian Exp $
+# $OpenBSD: slaacd,v 1.2 2017/06/03 10:00:29 florian Exp $
 
-daemon="/usr/sbin/slaacd"
+daemon="/sbin/slaacd"
 
 . /etc/rc.d/rc.subr