Add option to disable IPv6 tethering

Partially addresses #6, #20.

mobile/src/main/java/be/mygod/vpnhotspot/TetheringService.kt

@@ -49,6 +49,7 @@ class TetheringService : IpNeighbourMonitoringService(), VpnMonitor.Callback {
                     // system tethering already has working forwarding rules
                     // so it doesn't make sense to add additional forwarding rules
                     val routing = Routing(upstream, downstream).rule().forward().masquerade().dnsRedirect(dns)
+                    if (app.pref.getBoolean("service.disableIpv6", false)) routing.disableIpv6()
                     routings[downstream] = routing
                     if (!routing.start()) failed = true
                 } catch (e: SocketException) {

mobile/src/main/java/be/mygod/vpnhotspot/net/Routing.kt

@@ -47,6 +47,12 @@ class Routing(val upstream: String?, private val downstream: String, ownerAddres
         return this
     }
 
+    fun disableIpv6(): Routing {
+        startScript.add("echo 1 >/proc/sys/net/ipv6/conf/$downstream/disable_ipv6")
+        stopScript.add("echo 0 >/proc/sys/net/ipv6/conf/$downstream/disable_ipv6")
+        return this
+    }
+
     /**
      * Since Android 5.0, RULE_PRIORITY_TETHERING = 18000.
      * This also works for Wi-Fi direct where there's no rule at 18000.