Rename debian.interfaces to ifupdown

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>

CHANGELOG.md

@@ -6,6 +6,10 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ## v4.6.5, unreleased
 
+### Changed
+
+- Renamed debian.interfaces overlay to ifupdown
+
 ## v4.6.4, 2025-09-05
 
 ### Added