* querySubstitutablePathInfo: work properly when run via the daemon.

* --dry-run: print the paths that we don't know how to build/substitute.

src/libstore/worker-protocol.hh

@@ -33,6 +33,7 @@ typedef enum {
     wopQueryDeriver = 18,
     wopSetOptions = 19,
     wopCollectGarbage = 20,
+    wopQuerySubstitutablePathInfo = 21,
 } WorkerOp;