Make all terminal access/printing functions use (x, y) co-ordinates

This is an extremely high-risk change. I suspect that (row, col) instead of (x, y) would be of somewhat lower risk.

I just checked into this; it seems a likely a way to
(1) introduce bugs and
(2) piss off variant maintainers.

(x,y) is by far the less common ordering both in declarations and uses. It is limited to the various Term_*() functions; everything else is in (row, col) "term-style" orientation.

$ grep " x[0-9]*,.* y" *h | wc
      20     207    1327
Includes 6 false positives; this is used only in z-term.h

$ grep " y[0-9]*,.* x" *h |  grep -v z-term.h | wc
      43     359    2673

$ grep row.*col *h |wc
      12     108     744

Making the change would make it extremely difficult for other variants to reintegrate with V.