Some more conio support

The Atari Lynx target lacks conio (except for cgetc, which does not work correctly as reported in issue #959).

It seems to me that all building blocks for conio are already there because we have functions like tgi_outtextxy, tgi_setcolor and tgi_clear.
What is missing to have at least gotoxy and cputc?

We may just need two ZP locations for x and y cursor position and then we could just leverage on tgi_outtextxy to perform cputc and maybe cprintf.