add lowlevel tandy dac implementation

git-svn-id: http://svn.code.sf.net/p/dosbox/code-0/dosbox/trunk@3431 ed541006-0bf7-43e9-8c4d-6fc63c346d47

Conflicts:
	include/hardware.h
	src/hardware/tandy_sound.cpp
	src/ints/bios.cpp
reordering.

git-svn-id: http://svn.code.sf.net/p/dosbox/code-0/dosbox/trunk@3864 ed541006-0bf7-43e9-8c4d-6fc63c346d47
README.md: update

Signed-off-by: Ulrich Hecht <ulrich.hecht@gmail.com>