dnscontrol: install shell completions

DNSControl has built-in shell completions as described
in [1]. They can be installed on disk and will be loaded by bash-completion
or zsh.

[1]: https://docs.dnscontrol.org/getting-started/getting-started#id-1.1.-shell-completion

pkgs/applications/networking/dnscontrol/default.nix

@@ -1,4 +1,4 @@
-{ lib, buildGoModule, fetchFromGitHub, testers, dnscontrol }:
+{ lib, buildGoModule, fetchFromGitHub, installShellFiles, testers, dnscontrol }:
 
 buildGoModule rec {
   pname = "dnscontrol";
@@ -13,10 +13,18 @@ buildGoModule rec {
 
   vendorHash = "sha256-kmnV1W0HGlxFZYYUeUd9D/zOabUhM5kDoTZTnRYJ2sM=";
 
+  nativeBuildInputs = [ installShellFiles ];
+
   subPackages = [ "." ];
 
   ldflags = [ "-s" "-w" "-X=main.version=${version}" ];
 
+  postInstall = ''
+    installShellCompletion --cmd dnscontrol \
+      --bash <($out/bin/dnscontrol shell-completion bash) \
+      --zsh <($out/bin/dnscontrol shell-completion zsh)
+  '';
+
   preCheck = ''
     # requires network
     rm pkg/spflib/flatten_test.go pkg/spflib/parse_test.go