add rudimentary l2tp service module

modules/ppp/l2tp.nix

@@ -0,0 +1,63 @@
+{
+  liminix
+, lib
+, ppp
+, pppoe
+, writeAshScript
+, writeText
+, serviceFns
+, xl2tpd
+} :
+{ lns, ppp-options }:
+let
+  inherit (liminix.services) longrun;
+  name = "${lns}.l2tp";
+  ip-up = writeAshScript "ip-up" {} ''
+    . ${serviceFns} 
+    (in_outputs ${name}
+     echo $1 > ifname
+     echo $2 > tty
+     echo $3 > speed
+     echo $4 > address
+     echo $5 > peer-address
+     echo $DNS1 > ns1
+     echo $DNS2 > ns2
+    )
+    echo >/proc/self/fd/10
+  '';
+  ip6-up = writeAshScript "ip6-up" {} ''
+    . ${serviceFns} 
+    (in_outputs ${name}
+     echo $4 > ipv6-address
+     echo $5 > ipv6-peer-address
+    )
+    echo >/proc/self/fd/10
+  '';
+  ppp-options' = ppp-options ++ [
+    "ip-up-script" ip-up
+    "ipv6-up-script" ip6-up
+    "ipparam" name
+    "nodetach"
+    "usepeerdns"
+    "logfd" "2"
+  ];
+  conf = writeText "xl2tpd.conf" ''
+    [lac upstream]
+    lns = ${lns}
+    require authentication = no
+    pppoptfile = ${writeText "ppp-options" ppp-options'}
+    autodial = yes
+    redial = yes
+  '';
+  control = "/run/xl2tpd/control-${name}";
+in
+longrun {
+  inherit name;
+  run = ''
+    . ${serviceFns} 
+    mkdir -p /run/xl2tpd
+    touch ${control}
+    ${xl2tpd}/bin/xl2tpd -D -p /run/xl2tpd/${name}.pid -c ${conf} -C ${control} 
+  '';
+  notification-fd = 10;
+}