units: Simplify container getty handling

units: Simplify container getty handling

Let's remove the baud settings for the container getty units since
they don't have any effect there anyway. On top of that, when we're
dealing with container TTYs, we can handle all the setup involved
ourselves so let's prevent agetty/login from touching the container
tty at all.

One example where this helps is that it actually makes disabling
TTYVHangup have an effect since before, login would unconditionally
call vhangup() on the tty.