lab factory missed exit of `ipmitool` child process

We see this in the log:

Mar 27 01:55:54.348 INFO [sock/1945] serial: "SOL session closed by BMC", name: factory-lab

And nothing about serial for sock after that. There is no message specifically about the process going away that I can see, but it is indeed gone:

jclulow@catacomb ~
$ ptree buildomat
7675   /opt/buildomat/lib/buildomat-factory-lab -f config-lab.toml -d lab.sqlite3
  7678   <defunct>
  7679   /usr/sbin/ipmitool -C 3 -I lanplus -H buskin.lom.eng.oxide.computer -U ...

A few things:

• we should log at least the pid of the ipmitool child process; if we are today, I could not see it in there.
• find out why we missed the process exiting; we clearly have not called wait on the child as it has not been reaped
• belt and suspenders, we should treat an emission from ipmitool of "SOL session closed by BMC" on a line by itself as the start of some timeout after which we should forcefully chuck out the process if we have heard nothing else from it