interfaces: exclude a few cases from PPPoEv6 negotiation

6to4 and 6rd is static, as well as static itself.  Tracking does not
work on a WAN.  This leaves SLAAC and DHCPv6 with SLAAC being more
likely to work mutually exclusive.  Long term for DHCPv6 we need to
do something else like a manual override.

src/etc/inc/interfaces.inc

@@ -1026,8 +1026,18 @@ function interface_ppps_configure($interface)
         }
     }
 
-    /* XXX should be limited to 'pppoev6' match but some ISPs seem to require both racing away */
-    $ipv6_mode = ($ifcfg['ipaddrv6'] ?? 'none') != 'none' ? 'enable' : 'disable';
+    $ipv6_mode = 'disable';
+
+    switch ($ifcfg['ipaddrv6'] ?? 'none') {
+        /* XXX should be limited to 'pppoev6' match but some ISPs seem to require both racing away */
+        case 'dhcp6':
+        case 'pppoev6':
+        case 'slaac':
+            $ipv6_mode = 'enable';
+            break;
+        default:
+            break;
+    }
 
     // Construct the mpd.conf file
     $mpdconf = <<<EOD