programs/ssh: use 'toKeyValue' for 'extraOptions'

A first step towards refactoring the module.

Unfortunately the config is not consistent in using comma-separated
values or repeated keys for lists, since the user can always add commas
by themselves, we should default to repeated keys in the generator.

modules/programs/ssh.nix

@@ -451,7 +451,13 @@ let
       ++ map (f: "  LocalForward" + addressPort f.bind + addressPort f.host) cf.localForwards
       ++ map (f: "  RemoteForward" + addressPort f.bind + addressPort f.host) cf.remoteForwards
       ++ map (f: "  DynamicForward" + addressPort f) cf.dynamicForwards
-      ++ mapAttrsToList (n: v: "  ${n} ${v}") cf.extraOptions
+      ++ [
+        (lib.generators.toKeyValue {
+          mkKeyValue = lib.generators.mkKeyValueDefault { } " ";
+          listsAsDuplicateKeys = true;
+          indent = "  ";
+        } cf.extraOptions)
+      ]
     );
 
 in