Fix possible underflow conditions in gettimeleft()

If a non-'T' exempt user had already used more time today than their security level allows, their timeleft would be computed as a negative value due to integer underflow. Since the return value of this function is assigned to a ulong (timeleft), this becomes a large positive number. Cap the floor of the computed time left at 0.

Also fix the potential for underflow that could occur if the system clock changes while a user is online and 'now' becomes greater than 'starttime'.