Sync OpenBSD patchset 1072:

Add a simple form of output rate limiting by counting the number of
certain C0 sequences (linefeeds, backspaces, carriage returns) and if it
exceeds a threshold (current default 50/millisecond), start to redraw
the pane every 100 milliseconds instead of making each change as it
comes. Two configuration options - c0-change-trigger and
c0-change-interval.

This makes tmux much more responsive under very fast output (for example
yes(1) or accidentally cat'ing a large file) but may not be perfect on
all terminals and connections - feedback very welcome, particularly
where this change has a negative rather than positive effect (making it
off by default is a possibility).

After much experimentation based originally on a request Robin Lee
Powell (which ended with a completely different solution), this idea
from discussion with Ailin Nemui.

options-table.c

@@ -464,6 +464,21 @@ const struct options_table_entry window_options_table[] = {
 	  .default_num = 1
 	},
 
+
+	{ .name = "c0-change-trigger",
+	  .type = OPTIONS_TABLE_NUMBER,
+	  .default_num = 50,
+	  .minimum = 0,
+	  .maximum = USHRT_MAX
+	},
+
+	{ .name = "c0-change-interval",
+	  .type = OPTIONS_TABLE_NUMBER,
+	  .default_num = 100,
+	  .minimum = 1,
+	  .maximum = USHRT_MAX
+	},
+
 	{ .name = "clock-mode-colour",
 	  .type = OPTIONS_TABLE_COLOUR,
 	  .default_num = 4