illumos gate

We just uncovered a bug with tmux that turns out to be because its implementation of forkpty pushes the tty-related streams modules onto the slave, but when libc is XPG4 or above (__xpg4 is set), so do we.

This breaks the tty arbitrarily (we can't know its size, at the least).

We've talked in the past about implementing forkpty, doing so, and doing it properly, seems like a nice way to save people from having to know about this.