stty: add the ability to set the tty drainwait

[kevans]

Summary

drainwait is the only configurable bit left in comcontrol(8) after
4b4cf0d52c43e ("tty: Retire zombie dtrwait"). drainwait has a very
simple implementation in the tty layer, so it both makes little sense to
try and remove it and makes plenty of sense as something that stty(1)
can work with. Let's move it over with an eye towards retiring
comcontrol(8) in 16.0.