Make TLE detection for interactive problems more robust.

[meisterT]

While we plan to merge runguard and runpipe (see https://docs.google.com/document/d/1WZRwdvJUamsczYC7CpP3ZIBU8xG6wNqYqrNJf7osxYs/edit#heading=h.i7kgdnmw8qd7), this is a "minimal" change to signal (in the literal meaning of the word) to runpipe that runguard killed the program because of a timelimit.

[eldering]

I also pushed a small documentation fix commit.