Added user configurable frequency offset for surroundopl.

[PalMusicFan]

Hello!

Please refer to the issue of "User configurable FREQ_OFFSET? #70 ". For some music tracks, perhaps it is necessary to set a surroundopl frequency offset value greater than 128 in order to avoid "distortion" of certain voice. So I think a user configurable frequency offset for surroundopl is pretty useful.

I have added the feature of user configurable freq offset. Now the default frequency offset value is still defined by FREQ_OFFSET, which is 128. Frontend developers could set a new frequency offset value with something like:
opl->set_offset(cfg.offset);

[PalMusicFan]

This change is compatible with current frontends (at least current adplay-unix).
This is a demo of the new frontend of adplay-unix:
https://github.com/PalMusicFan/adplay-unix/tree/user-configurable-freq-offset
I have added a new parameter of "--offset" (or "-w", sorry I have no idea which character should be used for this, so I just simply chose the next unoccupied one) to the player and the default offset value is also 128.

[binarymaster]

Many thanks!