dnsmasq: pass through config file as an option

So that the pihole module reuse it. pihole-ftl is a fork of dnsmasq and its
configuration file is compatible.

nixos/modules/services/networking/dnsmasq.nix

@@ -115,6 +115,12 @@ in
         '';
       };
 
+      configFile = lib.mkOption {
+        type = lib.types.package;
+        default = dnsmasqConf;
+        internal = true;
+      };
+
     };
 
   };
@@ -172,7 +178,7 @@ in
       serviceConfig = {
         Type = "dbus";
         BusName = "uk.org.thekelleys.dnsmasq";
-        ExecStart = "${dnsmasq}/bin/dnsmasq -k --enable-dbus --user=dnsmasq -C ${dnsmasqConf}";
+        ExecStart = "${dnsmasq}/bin/dnsmasq -k --enable-dbus --user=dnsmasq -C ${cfg.configFile}";
         ExecReload = "${pkgs.coreutils}/bin/kill -HUP $MAINPID";
         PrivateTmp = true;
         ProtectSystem = true;