Change the xterm-keys option to default to on, so that tmux will

generate xterm(1) escape sequences for function keys with modifiers.

With the option off most of these keys are ignored by default, except
for ctrl + arrows which use a variant that nothing else seems to use and
I don't remember why we chose. The xterm escape sequences are now the
most common.

Prompted by a question from mpi@.

options-table.c

@@ -896,7 +896,7 @@ const struct options_table_entry options_table[] = {
 	{ .name = "xterm-keys",
 	  .type = OPTIONS_TABLE_FLAG,
 	  .scope = OPTIONS_TABLE_WINDOW,
-	  .default_num = 0
+	  .default_num = 1
 	},
 
 	{ .name = NULL }