sd-bus: sync kdbus.h (ABI break)

sd-bus: sync kdbus.h (ABI break)

After some reconsideration, we decided to move the binary protocol
back to 64-bit wide UIDs and GIDs. After all, it should be possible
to redefine [gu]id_t to uint64_t and things should continue to
work. As we want to avoid such data types in kdbus.h, let's move
back to 64-bit values and be safe.

In sd-bus, we have to do a translation between uint64_t and gid_t
now for supplementary gids.

Some inline comments have also been updated in kdbus upstream.