Remove uses of the "merge" option attribute

It's redundant because you can (and should) specify an option type, or
an apply function.

lib/options.nix

@@ -66,6 +66,9 @@ rec {
          then head list
     else throw "Cannot merge values.";
 
+
+  /* Obsolete, will remove soon.  Specify an option type or apply
+     function instead.  */
   mergeTypedOption = typeName: predicate: merge: list:
     if all predicate list then merge list
     else throw "Expect a ${typeName}.";

lib/types.nix

@@ -71,6 +71,12 @@ rec {
       merge = lib.concatStringsSep "\n";
     };
 
+    commas = mkOptionType {
+      name = "string";
+      check = builtins.isString;
+      merge = lib.concatStringsSep ",";
+    };
+
     envVar = mkOptionType {
       name = "environment variable";
       inherit (string) check;