Only open one event channel interface per process/domain

In userspace opening more than one interface is a good way to lose
events.

We also don't do anything on close(), to avoid a use-after-free.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
unix: make the Unix_activations.after interface first-class

The previous implementation could lose events.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Release 1.0.0, update CHANGES

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #5 from djs55/userspace-singleton2

Release 1.0.0