autumn 2022 bugfixes

• Fix "DNS issues", a firewall ruleset with a domain name lead to 100% CPU usage
  (reported by fiftyfourthparallel on
  https://forum.qubes-os.org/t/mirage-firewall-0-8-2-broken-new-users-should-install-0-8-1/14566,
  re-reported by @palainp in #158, fixed by @hannesm in mirage/mirage-nat#48
  (release 3.0.1)) - underlying issue was a wrong definition of is_port_free
  (since 3.0.0, used since mirage-qubes-firewall 0.8.2).
• Fix "crash on downstream vm start", after more than 64 client VMs have been
  connected and disconnected with the qubes-mirage-firewall (reported by @xaki23
  in #155, fixed by @hannesm in #161) - underlying issue was a leak of xenstore
  watchers and a hard limit in xen on the amount of watchers
• Fix "detach netvm fails" (reported by @rootnoob in #157, fixed by @palainp
  in mirage/mirage-net-xen#105 (release 2.1.2)) - underlying issue was that the
  network interface state was never set to closed, but directly removed
• Fix potential DoS in handling DNS replies (#162 @hannesm)
• Avoid potential forever loop in My_nat.free_udp_port (#159 @hannesm)
• Assorted code removals (#161 @hannesm)
• Update to dns 6.4.0 changes (#154, @hannesm)