lib: add option parameter to make default additive

It's sometimes the case that your option's default value should be additive such
that setting it results in the default value being merged with whatever was set.

Previously, you had to add the default value in the implementation which might
be many lines away from the option declaration and is generally an odd pattern.

This is a lot clearer, simpler and doesn't add much complexity.

If this becomes used more widely, it should be rendered in the generated options'
manual and nixos-search.

lib/modules.nix

@@ -780,9 +780,11 @@ let
   evalOptionValue = loc: opt: defs:
     let
       # Add in the default value for this option, if any.
-      defs' =
-          (optional (opt ? default)
-            { file = head opt.declarations; value = mkOptionDefault opt.default; }) ++ defs;
+      defs' = (optional (opt ? default) {
+        file = head opt.declarations;
+        value = (if opt.additive or false then id else mkOptionDefault) opt.default;
+      })
+      ++ defs;
 
       # Handle properties, check types, and merge everything together.
       res =

lib/options.nix

@@ -85,6 +85,8 @@ rec {
     visible ? null,
     # Whether the option can be set only once
     readOnly ? null,
+    # Whether the option default will be merged or overridden when the option is set
+    additive ? false
     } @ attrs:
     attrs // { _type = "option"; };