mixer: Implement non-linear time scale for fading.

mixer: Implement non-linear time scale for fading.

For some sound hardware, changing the volume channel has little
effect for small values but even an increment by one is audible for
high values. For such hardware it is desirable to scale the volume
in a non-linear way.

This patch implements this feature. The new --fade-exponent receives
an argument to control the level of non-linearity. The default value
of zero results in old behavior: linear scaling.

For the implementation we need to compute (the inverse of) t ->
t^alpha, where alpha is between zero and one. This requires the use
of exp(3) and log(3), so we need to include <math.h> and link with -lm.