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.

autoinstall/user-data

@@ -14,8 +14,9 @@ autoinstall:
   # the imaging LAN) stays without one. No static 172.16.9.1 here: matched
   # by name it would land on EVERY NIC, the GE one included, and the box
   # would answer as 172.16.9.1 on the corporate network. First boot's
-  # playbook replaces this file with the two-NIC config pinned by MAC
-  # (pxe_nic_mac / corp_nic_mac in playbook/site-vars.yml). optional: the
+  # playbook replaces this file with the imaging NIC pinned by MAC
+  # (pxe_nic_mac in playbook/site-vars.yml, or picked by its link when
+  # empty; corp_nic_mac optional). optional: the
   # install and boot never wait for a lease (air-gapped is fine).
   network:
     version: 2
@@ -139,9 +140,12 @@ autoinstall:
         if mount "$CIDATA_DEV" /mnt/usb; then
           if [ -f /mnt/usb/playbook/pxe_server_setup.yml ]; then
             cd /mnt/usb/playbook
+            rm -f "$STATE/nic-hint"
             ansible-playbook -i localhost, -c local pxe_server_setup.yml
             RC=$?
             WHY="ansible-playbook exit code $RC"
+            # pxe-pick-nic leaves the reason it could not pick the imaging NIC
+            [ "$RC" -ne 0 ] && [ -s "$STATE/nic-hint" ] && WHY="$(head -1 "$STATE/nic-hint")"
             cd /
           else
             WHY="no playbook/pxe_server_setup.yml on CIDATA"