Give jo/jovolterm fallback file

[Chris Warburton]

Commands like jo, jovol, etc. try to contact debian.local, but this
isn't always up to date with the actual IP address. We already allow
per-invocation fallbacks, with the JO_HOST env var.

We should add another fallback, which looks for a file like ~/.johost
and tries using its contents as an IP address. This way, we can write to
that file once and the commands will "just work", at least until a
different IP gets picked.