retirement: expand on vmtouch experiment

I had segfaults on nwipe on window resizes, hopefully that will help.

howto/retire-a-host.md

@@ -63,7 +63,7 @@ To wipe disks on servers without a serial console or management
 interface, you need to be a little more creative. We do this with the
 `nwipe(1)` command, which should be installed before anything:
 
-    apt install nwipe
+    apt install nwipe vmtouch
 
 Run in a screen:
 
@@ -101,12 +101,16 @@ When you return:
         export SHELL=/tmp/root/sh &&
         exec screen -s $SHELL
 
-    TODO: the above eventually failed to make busybox survive the
-    destruction, probably because it got evicted from RAM and couldn't
-    be found in swap again (as *that* was destroyed too). We should
-    try using [vmtouch](https://hoytech.com/vmtouch/) with something like `vmtouch -dl
-    /tmp/root/sh` next time, although that is only [available in buster
-    and later](https://tracker.debian.org/pkg/vmtouch).
+ 2. lock down busybox and screen in memory
+
+        vmtouch -dl /usr/bin/screen /bin/busybox /tmp/root/sh /usr/sbin/nwipe
+
+    TODO: the above aims at making busybox survive the destruction, so
+    that it's cached in RAM. It's unclear if that actually works,
+    because typically SSH is also busted and needs a lot more to
+    bootstrap, so we can't log back in if we lose the
+    console. Ideally, we'd run this in a serial console that would
+    have more reliable access... See also [vmtouch](https://hoytech.com/vmtouch/).
 
  2. kill all processes but the SSH daemon, your SSH connection and
     shell. this will vary from machine to machine, but a good way is