bus: sync with kdbus.git

Sync up with recent kdbus changed:
 * several ioctls gained .size and .items members (but still unused)
 * CMD_SEND gained its own ioctl structure
 * several members of kdbus_msg were dropped as they were only used during
   SEND, not during RECV etc.
 * CMD_RECV and CMD_SEND now share a kdbus_reply member which contains the
   offset and size of the returned message.