Strictly check the arguments to mkOption

And fix various instances of bad arguments.

lib/options.nix

@@ -11,17 +11,18 @@ with import ./strings.nix;
 rec {
 
   isOption = lib.isType "option";
-  mkOption = attrs: attrs // {
-    _type = "option";
-    # name (this is the name of the attributem it is automatically generated by the traversal)
-    # default (value used when no definition exists)
-    # example (documentation)
-    # description (documentation)
-    # type (option type, provide a default merge function and ensure type correctness)
-    # merge (function used to merge definitions into one definition: [ /type/ ] -> /type/)
-    # apply (convert the option value to ease the manipulation of the option result)
-    # options (set of sub-options declarations & definitions)
-  };
+  mkOption =
+    { default ? null # Default value used when no definition is given in the configuration.
+    , defaultText ? null # Textual representation of the default, for in the manual.
+    , example ? null # Example value used in the manual.
+    , description ? null # String describing the option.
+    , type ? null # Option type, providing type-checking and value merging.
+    , apply ? null # Function that converts the option value to something else.
+    , internal ? null # Whether the option is for NixOS developers only.
+    , visible ? null # Whether the option shows up in the manual.
+    , options ? null # Obsolete, used by types.optionSet.
+    } @ attrs:
+    attrs // { _type = "option"; };
 
   mkEnableOption = name: mkOption {
     default = false;

lib/types.nix

@@ -201,6 +201,9 @@ rec {
       name = /* builtins.trace "types.optionSet is deprecated; use types.submodule instead" */ "option set";
     };
 
+    # Augment the given type with an additional type check function.
+    addCheck = elemType: check: elemType // { check = x: elemType.check x && check x; };
+
   };
 
 }