strongswan module: make it work with ipsec l2tp

l2tp saves its secrets into /etc/ipsec.d but strongswan would not read
them. l2tp checks for /etc/ipsec.secrets includes /etc/ipsec.d and if
not tries to write into it.

Solution:
Have the strongswan module create /etc/ipsec.d and /etc/ipsec.secrets
when networkmanager_l2tp is installed.
Include /etc/ipsec.secrets in
/nix/store/hash-strongswan/etc/ipsec.secrets so that it can find l2tp
secrets.

Also when the ppp 'nopeerdns' option is used, the DNS resolver tries to
write into an alternate file /etc/ppp/resolv.conf. This fails when
/etc/ppp does not exist so the module creates it by default.

nixos/modules/services/networking/networkmanager.nix

···
 
       preStart = ''
         mkdir -m 700 -p /etc/NetworkManager/system-connections
+        mkdir -m 700 -p /etc/ipsec.d
         mkdir -m 755 -p ${stateDirs}
       '';
     };

nixos/modules/services/networking/strongswan.nix

···
       ${caConf}
     '';
 
-  strongswanConf = {setup, connections, ca, secrets, managePlugins, enabledPlugins}: toFile "strongswan.conf" ''
+  strongswanConf = {setup, connections, ca, secretsFile, managePlugins, enabledPlugins}: toFile "strongswan.conf" ''
     charon {
       ${if managePlugins then "load_modular = no" else ""}
       ${if managePlugins then ("load = " + (concatStringsSep " " enabledPlugins)) else ""}
       plugins {
         stroke {
-          secrets_file = ${ipsecSecrets secrets}
+          secrets_file = ${secretsFile}
         }
       }
     }
···
     };
   };
 
-  config = with cfg; mkIf enable {
+
+  config = with cfg;
+  let
+    secretsFile = ipsecSecrets cfg.secrets;
+  in
+  mkIf enable
+    {
+
+    # here we should use the default strongswan ipsec.secrets and
+    # append to it (default one is empty so not a pb for now)
+    environment.etc."ipsec.secrets".source = secretsFile;
+
     systemd.services.strongswan = {
       description = "strongSwan IPSec Service";
       wantedBy = [ "multi-user.target" ];
···
       wants = [ "keys.target" ];
       after = [ "network-online.target" "keys.target" ];
       environment = {
-        STRONGSWAN_CONF = strongswanConf { inherit setup connections ca secrets managePlugins enabledPlugins; };
+        STRONGSWAN_CONF = strongswanConf { inherit setup connections ca secretsFile managePlugins enabledPlugins; };
       };
       serviceConfig = {
         ExecStart  = "${pkgs.strongswan}/sbin/ipsec start --nofork";
       };
+      preStart = ''
+        # with 'nopeerdns' setting, ppp writes into this folder
+        mkdir -m 700 -p /etc/ppp
+      '';
     };
   };
 }

pkgs/tools/networking/network-manager/l2tp.nix

···
   ];
 
   enableParallelBuilding = true;
-  configureFlags = [
-    "--with-gnome=${if withGnome then "yes" else "no"}"
-    "--localstatedir=/var"
-  ] ;
-
 
   meta = with stdenv.lib; {
     description = "L2TP plugin for NetworkManager";
     inherit (networkmanager.meta) platforms;
-    homepage = http://github.com/nm-l2tp/network-manager-l2tp;
+    homepage = https://github.com/nm-l2tp/network-manager-l2tp;
     license = licenses.gpl2;
     maintainers = with maintainers; [ abbradar obadz ];
   };

pkgs/tools/networking/strongswan/default.nix

···
          "--enable-sqlite" ]
     ++ optional enableNetworkManager "--enable-nm";
 
+  postInstall = ''
+    # this is needed for l2tp
+    echo "include /etc/ipsec.secrets" >> $out/etc/ipsec.secrets
+  '';
+
   NIX_LDFLAGS = "-lgcc_s" ;
 
   meta = {