NICs: pick the imaging port at install, GE can go into any other port

Sites no longer need either MAC in site-vars.

pxe_nic_mac empty: before anything changes, the playbook runs
pxe-pick-nic --auto. It takes the one wired port with a link, accepts it
only if pxe-nic-check calls it isolated (no LLDP/CDP/802.1X, no other
DHCP server, at most 10 hosts heard in 65 s), and pins it. The choice is
saved in /etc/pxe-server/nic-pick.conf and never re-guessed at boot.
When it cannot tell (no link, two links, not isolated) nothing changes,
and the console banner says why and to run sudo pxe-pick-nic: pull the
imaging cable and plug it back in, and the port that blinked is the
imaging one. Both helpers are installed before the pick, so that works
after a refused first boot. The site's corp domains and allowed
switches are passed to the check.

corp_nic_mac empty: every other wired port (en*, eth*) is a catch-all
DHCP client, so GE can be plugged in at install or later.
- dnsmasq stays pinned with interface=pxe0. A new check refuses any
  dhcp-range outside the imaging subnet, and proxy ranges.
- The catch-all ports ignore offers from 172.16.9.1.
- UFW allows SSH and 9009 by source on any interface, and keeps those
  rules when the old any-interface rules are removed.
- pxe_nic_name must not look like en*/eth* in this mode.

pxe-netplan-apply.sh takes 3 or 5 args. Without a pinned GE NIC it:
- renames stray names (the old pxe0 after a re-pick, a leftover corp0)
  to enx<mac>, so the catch-all takes them, and restores them on revert;
- checks that every NIC that had a DHCP address before has one after,
  tracked by MAC.

With both MACs set (the WJ server), netplan, the dnsmasq pin and UFW
render byte-for-byte as before.

Tested in the pxetest VMs:
- Fresh install with no MACs and the GE link down: the imaging port was
  picked and pinned as pxe0 with 172.16.9.1, and dnsmasq bound DHCP to
  pxe0 only. GE plugged in afterwards got a lease, SSH worked, and the
  guard reported pxe0 isolated.
- --auto refuses with two links.
- The interactive pick works by pulling and replugging the link.
- A 3-arg re-apply is UNCHANGED.

Unit tests (fake sysfs) cover the re-pick and pinned-to-catch-all renames.

docs/pxe-nic-guard.md

@@ -32,7 +32,11 @@ Sources: `playbook/pxe-server-helpers/`, installed by `pxe_server_setup.yml`
 `pxe_nic_allowed_switches`; `pxe_imaging_mac` / `pxe_ge_mac` are derived
 from the first two). The same MACs pin the NICs in netplan
 (`playbook/templates/netplan-pxe.yaml.j2`, names `pxe0` / `corp0`) and in
-`/etc/dnsmasq.d/pxe-nic-only.conf`.
+`/etc/dnsmasq.d/pxe-nic-only.conf`. With `pxe_nic_mac` empty the playbook
+picks the imaging NIC at install time with `pxe-pick-nic --auto`, which
+runs this check on the one port with a link (`--iface X --probe`, 65 s)
+and accepts only "isolated". With `corp_nic_mac` empty `ge_mac` is blank
+and the GE NIC is whichever other port holds a DHCP lease.
 
 ## How it decides