Support TTY in IOSwitchboard.

The idea is to let IOSwitchboard isolator to allocate the pty. The master end will be sent to the IOSwitchboard server while the slavePath will be sent to launch.cpp. launch.cpp will perform login_tty to set the control terminal of the process to the allocated tty. We probably don't want to use login_tty directly because it'll create a new session which is not necessary.

It is unclear if this will need to be added to launch.cpp so that a pty is allocated at the time that a task is first launched. Or if it makes sense to only allow pty allocation on-the-fly as we attach to a process (in which case it will need to be the responsibility of the logger in our new model).