Flip kern.tty_info_kstacks on by default

It's a useful debug aid for anyone using Ctrl-T today, and doesn't seem to be
widely known. So, enable it out of the box to help people find it.

It's a tunable and sysctl, so if you don't like it, it's easy to disable
locally.

If people really hate it, we can always flip it back.

Reported by: Daniel O'Connor