Defined behaviour when creating subscriber without read access

## Brief feature description

If a user app does not have read access to any shared memory segment and a subscriber object is created, which `takes()` in case of a publisher is sending data, the current behaviour is a segfault.

## Detailed information

* Expected behaviour is that no data will ever be received from subscribers if the user has no read access to the shared memory segment to which the publisher is writing
* This needs a runtime check in the discovery loop
* Hence, add `posix::User to BasePort`
* Move `SegmentMapping` class to its own inl (segment_manager.hpp)