close the pipe with the listener process and wait for children to die.

adjust the logging level and forward it to every client process so they can do so too.

We may want to be able to run multiple listener process eventually, so the first step is to drop IMSG_CONTROLFD and keep the ctl stuff in the main process.

it's possibly a really useful tool to debug 9p servers, as it allows to manually craft messages, but it's just not there and I'd like to tag a 0.1 "soonish".