Fix build under clang and some warnings

Build problems fixed:
- void* pointer arithmetic which is a GNU extension (works as byte*)
- "variable size object may not be initialized" which is OK under GCC
- nullptr_t related error in json11 (it lacks 'operator <' in clang)

Warnings fixed:
- empty nested struct initializer { 0 } replaced by {}
- removed several unused lambda captures

src/stub_osd.cpp

@@ -112,7 +112,7 @@ int bind_stub(std::string bind_address, int bind_port)
 void run_stub(int peer_fd)
 {
     osd_any_op_t op;
-    osd_any_reply_t reply = { 0 };
+    osd_any_reply_t reply = {};
     void *buf = NULL;
     while (1)
     {