Add jul-app ABI/API and handle registration

Registration is done in the jul thread and the created jul-app are
attached to existing UST app.

At this stage, there is no need for the UST app to be linked to the JUL
app but in the future, by adding new features, this will be needed.

Enable/Disable one or all events are supported. Also, once a JUL
application registers, it's updated with the already enabled events.

Signed-off-by: David Goulet <dgoulet@efficios.com>