stty: fetch and print the configured drainwait

[kevans]

Summary

In the future, we will be removing comcontrol(8) in favor of just using
stty(1) for the one remaining feature, drainwait, which is configured
and implemented at the tty layer rather than per-driver. Start
collecting and rendering it for stty -a and stty -e, since the data
isn't really accessible otherwise unless it's at the default
kern.tty_drainwait.