shared/install: add a bit more quoting

shared/install: add a bit more quoting

When we are printing a valid unit name, quoting isn't necessary, because
unit names cannot contain whitespace or other confusing characters. In particular
if the unit name is prefixed by " unit " or something else that clearly
identifies the string as a unit name, quoting would just add unnecessary
noise. But when we're printing paths or invalid names, it's better to add
quotes for clarity.