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>

overlays/debug/internal/debug_test.go

@@ -143,8 +143,8 @@ node methods in addition to its fields.
   - udev.netname
   - systemd.netname
   - ifcfg
+  - ifupdown
   - NetworkManager
-  - debian.interfaces
   - wicked
   - ignition
 - Init: /sbin/init

overlays/debug/internal/nodes.conf

@@ -15,8 +15,8 @@ nodeprofiles:
     - udev.netname
     - systemd.netname
     - ifcfg
+    - ifupdown
     - NetworkManager
-    - debian.interfaces
     - wicked
     - ignition
     kernel: