compress: Overhaul the meaning of --aggressiveness.

Large values for the argument to --aggressiveless are next to useless
because they correspond to gain factors which are much too large. The
default value of four is already too large as it allows for a 16-fold
volume amplification.

This patch changes the meaning of the argument to be a multiplication
rather than a shift. Admissible values are restricted to the range
0..10, which allows for fine-grained control over the maximal gain
factor.

The maximal gain factor is four, so the inertia argument must not
exceed 14 to guarantee that no integer overflow can happen.