Force Session Timeout

While my team was building an extranet application for a client we came across
a slightly strange requirement from their InfoSec Audit team.

They want to forcibly log users out who have been active on the site for
longer than 12 hours making them re-authenticate.

This is not particularly user friendly - but audit teams tend to not be
particularly negotiable about ISO27000 requirements.

I had a look around and couldn't find anything which did specifically this -
so I took some time to write my first Drupal module.