nixos/zmap: init module

The module installs `zmap` globally and links the config files to
`/etc/zmap`, the default location of config files for zmap.

The package provides pretty much a sensitive default, custom configs can
be created like this:

```
{ lib, ... }:
{
environment.etc."zmap/blacklist.conf" = lib.mkForce {
text = ''
# custom zmap blacklist
0.0.0.0/0
'';
};
}
```

nixos/modules/module-list.nix

···
   ./programs/xonsh.nix
   ./programs/xss-lock.nix
   ./programs/yabar.nix
+  ./programs/zmap.nix
   ./programs/zsh/oh-my-zsh.nix
   ./programs/zsh/zsh.nix
   ./programs/zsh/zsh-autoenv.nix

nixos/modules/programs/zmap.nix

···
+{ pkgs, config, lib, ... }:
+
+with lib;
+
+let
+  cfg = config.programs.zmap;
+in {
+  options.programs.zmap = {
+    enable = mkEnableOption "ZMap";
+  };
+
+  config = mkIf cfg.enable {
+    environment.systemPackages = [ pkgs.zmap ];
+
+    environment.etc."zmap/blacklist.conf".source = "${pkgs.zmap}/etc/zmap/blacklist.conf";
+    environment.etc."zmap/zmap.conf".source = "${pkgs.zmap}/etc/zmap.conf";
+  };
+}