Preemption Signal Management

[Sargun Dhillon]

Andy pointed out that we need a mechanism to determine whether or
notifications are preempted. He suggested we use EPOLLPRI to indicate
whether or not notifications are preempted. My outstanding question is
whether or not we need to be able to get insight of what caused the
preemption, and to which notification.

In the past, Christian has suggested just background polling
notification IDs for validity, which is a fine mechanism to determine
that preemption has occurred. We could raise EPOLLPRI whenever a
notification has changed into the preempted state, but that would
require an O(n) operations across all outstanding notifications to
determine which one was preempted, and in addition, it doesn't give a
lot of information as to why the preemption occurred (fatal signal,
preemption?).

In order to try to break this into small parts, I suggest:
1. We make it so EPOLLPRI is raised (always) on preempted notifications
2. We allow the user to set a flag to "track" notifications. If they
specify this flag, they can then run a "stronger" ioctl -- let's say
SECCOMP_IOCTL_NOTIF_STATUS, which, if the flag was specified upon
receiving the notification will return the current state of the
notification and if a signal preempted it, it will always do that.

---
Alternatively (and this is my preference), we add another filter flag,
like SECCOMP_FILTER_FLAG_NOTIF_PREEMPT, which changes the behaviour
to:
1. Raise EPOLLPRI on preempted notifications
2. All preemption notifications must be cleared via
SECCOMP_IOCTL_NOTIF_RECV_STATUS.

Opinions?