face: Create the socket file in UnixStreamChannel world-writable.

Fixes a permission denied error when nfd is run as root and a non-root
application tries to communicate with it over a Unix face.

refs: #1272
Change-Id: I4ba60f9a9dfbec12d787424dff98f38b76b48cf0
diff --git a/daemon/face/unix-stream-channel.hpp b/daemon/face/unix-stream-channel.hpp
index bd0a81e..3bbc8a3 100644
--- a/daemon/face/unix-stream-channel.hpp
+++ b/daemon/face/unix-stream-channel.hpp
@@ -38,6 +38,14 @@
   typedef function<void(const std::string& reason)> ConnectFailedCallback;
 
   /**
+   * \brief UnixStreamChannel-related error
+   */
+  struct Error : public std::runtime_error
+  {
+    Error(const std::string& what) : std::runtime_error(what) {}
+  };
+
+  /**
    * \brief Create UnixStream channel for the specified endpoint
    *
    * To enable creation of faces upon incoming connections, one