grandenet: Fix up grandenet script when set ip6tables

The ip6tables-save output can contains several entries (*filter, *raw, for diferent tables), try to append re6stnet specific configuration on the right location w/o break the content. 

net-misc/re6stnet/files/grandenet

@@ -46,6 +46,7 @@ function configure {
     printf '%s\n' "${configoptions[@]}" >> "${configfile}"
 
     echo "
+*filter
 :FORWARD ACCEPT [0:0]
 :OUTPUT DROP [0:0]
 -A INPUT -p udp -m udp --dport 6696 -j ACCEPT
@@ -54,6 +55,7 @@ function configure {
 -A INPUT -p tcp -m tcp --dport 50005 -j ACCEPT
 -A OUTPUT -p udp -m udp --dport 6696 -j ACCEPT
 -A OUTPUT -p udp -m udp --dport 326 -j ACCEPT
+COMMIT
 " > "${configdir}/ip6tables.conf"
 
     read -p "Save configuration on an external device [Y/n]? " save
@@ -209,12 +211,8 @@ fi
 
 # firewall configuration
 if [ -r "${configdir}/ip6tables.conf" ] ; then
-    # remove line containing "COMMIT"
-    sed -i '/COMMIT/d' "${configdir}/current_ip6tables.conf"
     # add few rules
     cat "${configdir}/ip6tables.conf" >> "${configdir}/current_ip6tables.conf"
-    # putting "COMMIT" line back
-    echo "COMMIT" >> "${configdir}/current_ip6tables.conf"
     ip6tables-restore < "${configdir}/current_ip6tables.conf"
 else
     # accept ports needed for re6stnet