* Allow unprivileged users to run the garbage collector and to do

`nix-store --delete'.  But unprivileged users are not allowed to
  ignore liveness.
* `nix-store --delete --ignore-liveness': ignore the runtime roots as
  well.

src/libstore/worker-protocol.hh

@@ -25,6 +25,7 @@ typedef enum {
     wopAddIndirectRoot,
     wopSyncWithGC,
     wopFindRoots,
+    wopCollectGarbage,
 } WorkerOp;