DDS15 — DDS specification should be more precise on NATIVE defines

DDS leaves domainid, handle, and topic key to the dds vendor, each can define their own type for these. The specification has defines for them, but the default is long. If now a vendor uses for example a struct for the InstanceHandle_t, this leads to challenges when writing portable C++ code because the argument passing is different between a long (as value) or a fixed struct (const &). To my idea the dds specification should be more precise on the type, maybe fixed struct to achieve more portability of users code