Plumber both posts a service to /srv and sets a $plumbsrv environment
variable. Our libplumb no longer uses $plumbsrv and nothing else
does. It's a silly hack; rc doesn't update /env immediately, and
scripts, which for instance set up subrios, cannot rely on it to
clean up the plumber at the end.
Instead, add the option to specify a srvname, actually check for some
common errors and print a usage string.
Thanks to Ori for input and a preliminary patch.
