kdbus: drop KDBUS_ITEM_SIGMASK

kdbus: drop KDBUS_ITEM_SIGMASK

The sigset_t type is arch-dependent. We really don't want such types in
our kdbus API. Our CANCEL_FD provides a safe alternative, so use it.

If anyone wants SIGMASK support later on, we can always add it again. But
unless someone wants it, we will try hard to keep it out of kdbus.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>