Disable IPv6 via grub command line

Unfortunately disabling IPv6 via sysctl doesn't actually
disable it on all of the interfaces by default. Disabling
it via grub disables it entirely in the VM.

Helps with #454

util/install.sh

@@ -554,9 +554,9 @@ function vm_other {
     #    BLACKLIST=/etc/modprobe.d/blacklist
     #fi
     #sudo sh -c "echo 'blacklist net-pf-10\nblacklist ipv6' >> $BLACKLIST"
-
+    echo "Disabling IPv6"
     # Disable IPv6
-    if ! grep 'disable IPv6' /etc/sysctl.conf; then
+    if ! grep 'disable_ipv6' /etc/sysctl.conf; then
         echo 'Disabling IPv6'
         echo '
 # Mininet: disable IPv6
@@ -564,6 +564,13 @@ net.ipv6.conf.all.disable_ipv6 = 1
 net.ipv6.conf.default.disable_ipv6 = 1
 net.ipv6.conf.lo.disable_ipv6 = 1' | sudo tee -a /etc/sysctl.conf > /dev/null
     fi
+    # Since the above doesn't disable neighbor discovery, also do this:
+    if ! grep 'ipv6.disable' /etc/default/grub; then
+        sudo sed -i -e \
+        's/GRUB_CMDLINE_LINUX_DEFAULT="/GRUB_CMDLINE_LINUX_DEFAULT="ipv6.disable=1/' \
+        /etc/default/grub
+        sudo update-grub
+    fi
     # Disabling IPv6 breaks X11 forwarding via ssh
     line='AddressFamily inet'
     file='/etc/ssh/sshd_config'