More futureproofing hypotheticals

If you set COLNO larger than BUFSZ, few places cause a buffer overrun.
Add a new buffer size definition, COLBUFSZ, which is the larger of
COLNO and BUFSZ, used in places that care about a screen-wide string.

src/options.c

@@ -8307,7 +8307,7 @@ const char *str;
     char *s;
 
     if (!buf)
-        *(buf = (char *) alloc(BUFSZ)) = '\0';
+        *(buf = (char *) alloc(COLBUFSZ)) = '\0';
 
     if (!*str) {
         s = eos(buf);