* Pass various options to the worker so that flags like -K or -j work

in multi-user Nix (NIX-72).
* Client/worker: exchange a protocol version number for future
  compatibility.

src/libstore/worker-protocol.hh

@@ -5,8 +5,11 @@
 namespace nix {
 
 
-#define WORKER_MAGIC_1 0x6e697864
-#define WORKER_MAGIC_2 0x6478696e
+#define WORKER_MAGIC_1 0x6e697863
+#define WORKER_MAGIC_2 0x6478696f
+
+#define PROTOCOL_VERSION 0x101
+#define GET_PROTOCOL_MAJOR(x) ((x) & 0xff00)
 
 
 typedef enum {
@@ -28,6 +31,7 @@ typedef enum {
     wopExportPath,
     wopImportPath,
     wopQueryDeriver,
+    wopSetOptions,
 } WorkerOp;