nixos: add config.services.openssh.moduliFile option so that users can replace the default file from OpenSSH

The man page for ssh-keygen(1) has a section "MODULI GENERATION" that describes
how to generate your own moduli file. The following script might also be helpful:

 | #! /usr/bin/env bash
 |
 | moduliFiles=()
 |
 | generateModuli()
 | {
 |   ssh-keygen -G "moduli-$1.candidates" -b "$1"
 |   ssh-keygen -T "moduli-$1" -f "moduli-$1.candidates"
 |   rm "moduli-$1.candidates"
 | }
 |
 | for (( i=0 ; i <= 16 ; ++i )); do
 |   let bitSize="2048 + i * 128"
 |   generateModuli "$bitSize" &
 |   moduliFiles+=( "moduli-$bitSize" )
 | done
 | wait
 |
 | echo >moduli "# Time Type Tests Tries Size Generator Modulus"
 | cat >>moduli "${moduliFiles[@]}"
 | rm "${moduliFiles[@]}"

Note that generating moduli takes a long time, i.e. several hours on a fast
machine!

This patch resolves https://github.com/NixOS/nixpkgs/pull/5870.

nixos/modules/services/networking/ssh/sshd.nix

@@ -268,6 +268,16 @@ in
         };
       };
 
+      moduliFile = mkOption {
+        example = "services.openssh.moduliFile = /etc/my-local-ssh-moduli;";
+        type = types.path;
+        description = ''
+          Path to <literal>moduli</literal> file to install in
+          <literal>/etc/ssh/moduli</literal>. If this option is unset, then
+          the <literal>moduli</literal> file shipped with OpenSSH will be used.
+        '';
+      };
+
     };
 
     users.extraUsers = mkOption {
@@ -286,8 +296,10 @@ in
         description = "SSH privilege separation user";
       };
 
+    services.openssh.moduliFile = mkDefault "${cfgc.package}/etc/ssh/moduli";
+
     environment.etc = authKeysFiles ++ [
-      { source = "${cfgc.package}/etc/ssh/moduli";
+      { source = cfg.moduliFile;
         target = "ssh/moduli";
       }
       { text = knownHostsText;