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/messenger.cpp

@@ -544,7 +544,7 @@ json11::Json osd_messenger_t::read_config(const json11::Json & config)
     int done = 0;
     while (done < st.st_size)
     {
-        int r = read(fd, (void*)buf.data()+done, st.st_size-done);
+        int r = read(fd, (uint8_t*)buf.data()+done, st.st_size-done);
         if (r < 0)
         {
             fprintf(stderr, "Error reading %s: %s\n", config_path, strerror(errno));