So, we cannot use veth.$vmid.$ifid, and we cannot use vif.$vmid.$ifid,
because the latter seems to (sometimes) be used internally in some
way that races with our use.  Good news!  veth$vmid.$ifid seems to work.
This is ridiculous.

Something changed at Xen 4.16/Linux 5.15, where if the
net.bridge.bridge-nf-call-iptables sysctl is enabled (which we
do so that we can firewall/nat the control net bridge ifaces),
iptables rules apply to the expt net bridges as well.  This
seems to be new behavior, because although we set the default
global FORWARD chain policy to DROP, and do not create
iptables rules for expt net bridges, in Xen 4.11/Linux 5.4,
the bridge forwarded ip packets just fine.  In Xen 4.16/Linux
5.15, this behavior has changed, and ip traffic is not
forwarded unless we add this basic allow-all forwarding rule
for each expt bridge.

Something in Xen no longer allows our prior PV vif naming convention
(veth.$vmid.$ifid), and switching to theirs works (vif$vmid.$ifid);
make it so.  Keep backwards compat for prior Xen versions.

In this commit, we actually write the proper configuration to systemd-networkd
files instead of stopping systemd entirely.  Moreover, we grab the nameserver
configuration from systemd-resolved if the stub resolver is in use, and migrate
it to the xenbr.  All in the name of upstream compat/coexistence and avoiding
special hacks.

The upstream pygrub works fine on our standard Ubuntu images as of 4.16.

In one situation, we were getting our own MC packets echoed back to us and
we were accepting our JOIN reply messages as JOIN request messages and
sending replies to those. The situation deteriorated rapidly.

with 512 byte sectors.

{5484} boss.utah$ manage_instance terminate -h
Terminate an experiment
Usage:
manage_instance [-ds] [-t id] terminate <instance> [-eLX]
Options:
  -e       - Record the experiment as expired instead of terminated
  -L       - Take the experiment lock from whoever has it
  -X       - Only the experiment creator can terminate (legacy RPC)
Global Options:
  -d       - Debugging, do not fork into the background
  -s       - Silent (no email)
  -t id    - Webtask ID

older Linux implementations. Who knew that FreeBSD had a newer version
of EXTFS tools than Ubuntu 22?

Need some downtime between "mount" and "blkid" invocations, else blkid
returns nothing. I suspect busybox implementations are causing grief.