bug#2993: 23.0.92; posn-col-row wrong with line-spacing in terminals

[Nikolaj Schumacher]

posn-col-row uses the `line-spacing' value in terminals.  According to the
doc `line-spacing' only has an effect in window systems.

Steps to reproduce:

- (setq-default line-spacing 3)
- emacsclient -t
- Move to a non-trivial pos.
- M-: (posn-col-row (posn-at-point))

Something like this would help:

--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -934,7 +934,8 @@ and `event-end' functions."
             (y (/ (cdr pair) (+ (frame-char-height frame)
                                 (or (frame-parameter frame 'line-spacing)
                                      ;; FIXME: Why the `default'?
-                                    (default-value 'line-spacing)
+                                    (when (window-system frame)
+                                      (default-value 'line-spacing))
                                     0)))))
        (cons x y))))))