Rework score formula.

Rework score formula.

Currently the two scales for the num_played and the last_played
components of the score value are computed independently of each
other. There is, however, a natural link between the two scales:
a file with best possible num_played value (zero) and worst possible
last_played value (now) should receive the average score zero.

This patch employs this idea to rescale the two components. See the
new comment to compute_score() for details about the implementation.