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.

pkgs/tools/networking/strongswan/default.nix

@@ -76,6 +76,11 @@ stdenv.mkDerivation rec {
          "--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 = {