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/msgr_send.cpp

@@ -250,7 +250,7 @@ void osd_messenger_t::handle_send(int result, osd_client_t *cl)
             else
             {
                 iov.iov_len -= result;
-                iov.iov_base += result;
+                iov.iov_base = (uint8_t*)iov.iov_base + result;
                 break;
             }
         }