Refactor module scan frame counter

The scan code is one of the oldest and ugliest parts of the player. It uses
obscure logic to gather timing and volume data from the module prior to
actual replay. We'll try to rename and add meaning to some of the variables
in an attempt to make this code easier to maintain.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Change scan routine variable names

Use consistent naming for the scan routine variables to try to make that
piece of code less obscure.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Change module scan valid value offsets

Instead of using zero as uninitialized values and adding one to valid
values, use -1 as unitialized and remove a few increments and decrements
that make the code more confusing.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>