core: manage SetShowStatus through polkit

core: manage SetShowStatus through polkit

When writing docs for SD_BUS_VTABLE_CAPABILITY, I noticed that we have one use
of SD_BUS_VTABLE_CAPABILITY(CAP_SYS_ADMIN) in the tree. This is the default, so
it's not very useful to specify it. But if we're touching that, I think it's
better to use mac + polkit for this like for everything else.

We don't have a very good category for this, but I don't think it makes sense
to add a new one. I just reused the same as other similar calls.