Reset envelope wave and phase also for unmodified shapes

Double buffer DMA sound registers

This improves Mad_Max/Games/Lethal_Xcess_(STe).sndh and
Mad_Max/Games/Chambers_of_Shaolin_STe.sndh and most likely others.

Add --psg-mix=<empiric|linear> option (default linear)

Add BUILD_COMPILE variable

Now

	make BUILD_CC=m68k-elf-gcc

is the same as

	make BUILD_COMPILE=m68k-elf-

Tag disassembly with init, exit or play as comments

Link to media-sound/psgplay package for Gentoo Linux