refactor(mbyte.c): add const qualifiers

This only touches functions that do not return a pointer.
Also add a note about the differences between mb_head_off() and utf_head_off().

src/nvim/buffer.c

@@ -3327,7 +3327,7 @@ void maketitle(void)
       len = (int)STRLEN(buf_p);
       if (len > 100) {
         len -= 100;
-        len += (*mb_tail_off)(buf_p, buf_p + len) + 1;
+        len += mb_tail_off(buf_p, buf_p + len) + 1;
         buf_p += len;
       }
       STRCPY(icon_str, buf_p);