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