main: ensure tty is closed on deactivate

If plymouth doesn't get explicitly "activated" then when GDM tries to deactivate it, the deactivation request is a noop.

One aspect of being active, though is having ownership and control of the terminal. This happens immediately, even before a splash is shown.

The deactivate request needs to relinguish such control, unconditionally, since some display server is about to use the tty.

This commit fixes that.