[master] Add #define for length of prefix we send to v6 client script

RELNOTES

@@ -139,6 +139,11 @@ by Eric Young (eay@cryptsoft.com).
   Thanks to Marius Tomaschewski for the suggestion and proto-patch.
   [ISC-Bugs #29713]
 
+- Add a #define to specify the prefix length used when a client attempts
+  to configure an address.  This can be modified by editing includes/site.h.
+  By default it is set to 64.  While 128 might be a better choice it would
+  also be a change for currently running systems, so we have left it at 64.
+
 			Changes since 4.3.0rc1
 
 - None

client/dhc6.c

@@ -3851,11 +3851,8 @@ dhc6_marshall_values(const char *prefix, struct client_state *client,
 				      piaddr(addr->address),
 				      (unsigned) addr->plen);
 		} else {
-			/* Current practice is that all subnets are /64's, but
-			 * some suspect this may not be permanent.
-			 */
 			client_envadd(client, prefix, "ip6_prefixlen",
-				      "%d", 64);
+				      "%d", DHCLIENT_DEFAULT_PREFIX_LEN);
 			client_envadd(client, prefix, "ip6_address",
 				      "%s", piaddr(addr->address));
 		}

includes/site.h

@@ -284,7 +284,17 @@
    entail updating all the records at once, probably at start
    up. */
 #define DDNS_UPDATE_SLOW_TRANSITION
-   
+
+/* Define the default prefix length passed from the client to
+   the script when modifying an IPv6 IA_NA or IA_TA address.
+   The two most useful values are 128 which is what the current
+   specifications call for or 64 which is what has been used in
+   the past.  For most OSes 128 will indicate that the address
+   is a host address and doesn't include any on-link information.
+   64 indicates that the first 64 bits are the subnet or on-link
+   prefix. */
+#define DHCLIENT_DEFAULT_PREFIX_LEN 64
+
 /* Include definitions for various options.  In general these
    should be left as is, but if you have already defined one
    of these and prefer your definition you can comment the