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

Sync kdbus.h with upstream changes:

  * Two optional cancellation points where added for synchronously
    blocking KDBUS_CMD_SEND commands: A sigmask to change the mask
    of accepted signals before the task is put to sleep, and a
    generic file descriptor that can be written to, in order to cancel
    the command. Both methods are currently unused.

  * The KDBUS_CMD_CANCEL ioctl was removed. sd-bus was never using
    that command, so there's no change needed.

  * Some kerneldoc fixes