Enable the unix:// store on Windows

Windows now has some basic Unix Domain Socket support, see
https://devblogs.microsoft.com/commandline/af_unix-comes-to-windows/

Building `nix daemon` on Windows I've left for later, because the daemon
currently forks per connection but this is not an option on Windows. But
we can get the client part working right away.

src/libstore/indirect-root-store.hh

@@ -67,6 +67,9 @@ struct IndirectRootStore : public virtual LocalFSStore
      * The form this weak-reference takes is implementation-specific.
      */
     virtual void addIndirectRoot(const Path & path) = 0;
+
+protected:
+    void makeSymlink(const Path & link, const Path & target);
 };
 
 }