Mac: Start the move to the new APIs for launching applications in dw_exec(). The new APIs were introduced in Catalina (10.15), so we need to keep the old APIs for all older versions, so the warnings I was trying to eliminate remain. Also noticing that the console execution is still using xterm instead of Terminal.app, xterm existed in very old versions but Apple has since removed the X11.app and thus the xterm program. Will move to Terminal.app in a later commit.