patch 9.0.0040: use of set_chars_option() is confusing

Problem:    Use of set_chars_option() is confusing.
Solution:   Add "apply" argument to store the result or not.  Merge similar
            code.
diff --git a/src/proto/screen.pro b/src/proto/screen.pro
index de42c43..66fc0f3 100644
--- a/src/proto/screen.pro
+++ b/src/proto/screen.pro
@@ -55,5 +55,5 @@
 int number_width(win_T *wp);
 int screen_screencol(void);
 int screen_screenrow(void);
-char *set_chars_option(win_T *wp, char_u **varp);
+char *set_chars_option(win_T *wp, char_u **varp, int apply);
 /* vim: set ft=c : */