improved fading routines, fixed DAC sounds in VGM logs

- enforced creation of "dumps" folder for VGM/WAV logging
- added FadeOutMusic_Custom()
- added FadeOut struct to SMPS_CFG, used by DoFadeOut()
- added FadeOutSteps/Delay/VolAdd* settings to DefDrv.txt
- fixed PLAY_DAC/Mercs command
- added DAC bank support to preparser (fixes Zaxxon Motherbase 2000)
- added DoSpecialFade()
- renamed FADE_IN command to FADE_SPC
- added FADE_SPC subcommands