nixos/dnsmasq: allow custom package

Allow a custom package to be used for dnsmasq service.

nixos/modules/services/networking/dnsmasq.nix

@@ -4,7 +4,7 @@ with lib;
 
 let
   cfg = config.services.dnsmasq;
-  dnsmasq = pkgs.dnsmasq;
+  dnsmasq = cfg.package;
   stateDir = "/var/lib/dnsmasq";
 
   # True values are just put as `name` instead of `name=true`, and false values
@@ -53,6 +53,8 @@ in
         '';
       };
 
+      package = mkPackageOptionMD pkgs "dnsmasq" {};
+
       resolveLocalQueries = mkOption {
         type = types.bool;
         default = true;