install: introduce new DefaultInstance= field for [Install] sections

install: introduce new DefaultInstance= field for [Install] sections

The DefaultInstance= name is used when enabling template units when only
specifying the template name, but no instance.

Add DefaultInstance=tty1 to getty@.service, so that when the template
itself is enabled an instance for tty1 is created.

This is useful so that we "systemctl preset-all" can work properly,
because we can operate on getty@.service after finding it, and the right
instance is created.