What we really want is to either output to WM_NAME or stdout. If we want
just one single line, we do slstatus | head -n 1.

d-flag: There's no need for that, use the &-operator or fork+exec in a
        non-shell-context. In the latter case you get the PID for free.
v-flag: If you want to find out which version of a package is installed,
        consult your package manager. That's his job.

We also change the semantics of -v to only return the version
information.
There is now no need for usage() to exit with anything other than 1.