run: add a switch for specifiying the working directory of a service

run: add a switch for specifiying the working directory of a service

I find myself testing service management quite often with "systemd-run
-t /bin/bash". For that it is handy if the invoked shell would use the
working directory I am currently in. Hence introduce a shorthand for
that:

        $ systemd-run -dt /bin/bash

This will automatically insert a WorkingDirectory= property into the
transient service, pointing to the working directory of the caller.