When I first split the debian interfaces support from wwinit, I was unable to
figure out what it was called. I used the inelegant name "debian.interfaces" to
identify it; but I have since learned that it is called "ifupdown." I have
since learned that there is even a variant implementation, ifupdown-ng, that is
used by Alpine Linux.
This PR renames the overlay to ifupdown, but retains a copy of it at
debian.interfaces for backwards compatibility.
Signed-off-by: Jonathon Anderson <janderson@ciq.com>
