lib/generators: add mkValueString to mkKeyValueDefault generator

This means the generation of values can now be influenced, even down from e.g.
an INI generator.
Breaks the interface of `mkKeyValueDefault` to match its interface to other
generator functions. It might me sensible to rename `mkKeyValue` and
`mkKeyValueSet` to conform to the `toX`-style of generator functions.

lib/tests/misc.nix

@@ -201,7 +201,7 @@ runTests {
 # in alphabetical order
 
   testMkKeyValueDefault = {
-    expr = generators.mkKeyValueDefault ":" "f:oo" "bar";
+    expr = generators.mkKeyValueDefault {} ":" "f:oo" "bar";
     expected = ''f\:oo:bar'';
   };