patch 8.2.3320: some local functions are not static

Problem:    Some local functions are not static.
Solution:   Add "static".  Move snprintf() related code to strings.c.
            (Yegappan Lakshmanan, closes #8734)

src/channel.c

@@ -3025,7 +3025,7 @@ channel_has_readahead(channel_T *channel, ch_part_T part)
  * Return a string indicating the status of the channel.
  * If "req_part" is not negative check that part.
  */
-    char *
+    static char *
 channel_status(channel_T *channel, int req_part)
 {
     ch_part_T part;