Compute remaining session time correctly if time extension is active.

[marcus67]

Current behavior: When a time extension is active the remaining session time is always defined by the time extension.
Desired behavior: If a time is extension is activated for a user while he is not permitted to play the time extension starts at the current point of time. It may extend into a time period in which the user IS allowed to play. In these cases the remaining session time is not defined by the time extension but by the normal rules. C