In it's latest release, agetty will support reading the agetty.autologin
and login.noauth credentials, so let's make sure we import those in our
getty units so they're available to agetty to read.
If the tty arg is set to "-", agetty uses the stdin fd as the tty.
Let's pass the tty this way so that we keep an fd open to the tty
at all times. If all fd's to a tty are closed, the kernel might
reset the tty which we want to avoid.
We don't need two (and half) templating systems anymore, yay!
I'm keeping the changes minimal, to make the diff manageable. Some enhancements
due to a better templating system might be possible in the future.
For handling of '## ' — see the next commit.
