linecount: use pane_too_long() to break up line count

linecount: use pane_too_long() to break up line count

Rather than arbitrary numbers of lines to be counted before we take a
break and check for user activity, use pane_too_long() to decide when to
drop a mark, and when to take a break.

Also reduce the default and minimum timeout to 100ms which should help
interactivity a bit.

Signed-off-by: NeilBrown <neil@brown.name>