This patch implements a new event type, it will trigger whenever a
value becomes greater than user-specified threshold, it complements
the 'less-then' trigger type.
Also, let's implement the one-shot mode for the events, when set,
userspace will only receive one notification per crossing the
boundaries.
Now when both LT and GT are set on the same level, the event type
works as a cross event type: it triggers whenever a value crosses
the threshold from a lesser values side to a greater values side,
and vice versa.
We use the event types in an userspace low-memory killer: we get a
notification when memory becomes low, so we start freeing memory by
killing unneeded processes, and we get notification when memory hits
the threshold from another side, so we know that we freed enough of
memory.