kdbus.h: various updates of the API

As discussed in Berlin today, with Kay and Daniel
kdbus.h: since TASK_COMM_LEN is not exported, let's do dynamically sized comm strings again
kdbus: make the thing compile again
notify: explicitly set message payload type

kernel-generated messages always have KDBUS_PAYLOAD_NONE.
Revert "kdbus_internal.h: use size_t in kdbus_size_{set,get}_user"

We need to do this properly for arm.

This reverts commit 348ee95cc54d8ec7ff3e977ecfd4865df8389da2.
test: add build dependency to kdbus.h
kdbus.h: the structs for making EPs, busses and namespaces should be separate

They need different parameters, so let's split this up.
respect access flags for namespace control device node