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

In kdbus.h, the following details changed:

 * All commands gained a 'kernel_flags' field to report the flags supported
   by the driver. Before, this was done in the 'flags' field in a
   bidirectional way, which turned out to be a problem for the code in
   sd-bus, as many parts of it reuse the same ioctl struct more than once
   and consider them to be owned by userspace.

 * Name listings are now returned by a new struct instead of reusing struct
   kdbus_cmd_name for that matter. This way, we don't add more unneeded
   fields to it and make the API cleaner.

 * 'conn_flags' was renamed to 'flags' in struct kdbus_cmd_hello to make
   the API a bit more unified.