install: make "reenable" work with templated units

Before, "systemctl reenable getty@tty1.service" would fail with:
Failed to issue method call: File exists
To fix this, reimplement "reenable" explicitly as a disable followed by
an enable.
This is shorter and is how the man page documents its behavior.
units: do not special-case getty@tty1.service installation

Since a long while we can use "systemctl enable getty@tty1.service"
which does the right thing, so there's no need to abuse Alias=
for installation.