Add session limits support and PAM module

This is a large new feature to support time limited login sessions, limiting the amount of time a user can use the computer before they’re kicked off.

Currently, the only type of limit supported is use of the computer between a certain start and end time each day (the same each day). Additional limit types can more easily be added in future, but most will require some way of storing the length of previous sessions so that we know the cumulative active session time.