Update to the Bomb Lab

We've updated the Bomb Lab sources on the CS:APP site to address a problem that arises when students from previous semesters run their old bombs while the current instance of the lab is underway.

The Bomb Lab servers assign diffusions and explosions to Bomb IDs, rather than users, and Bomb IDs start over from scratch each term. Thus, if a student  who took the class last semester ran their old bomb while the lab was as underway this semester, then the explosions and diffusions from the old bomb would be incorrectly assigned to the current bomb with the same Bomb ID.

To address this, we've added a per-semester identifier, called $LABID,  to the Bomb Lab config file. Instructors can set this variable each term (for example $LABID="f12") to uniquely identify each offering. Any results from previous bombs with different $LABIDs are ignored.

Thanks for Prof. Godmar Bak, Virginia Tech, for pointing this out.