core: expose PrivateTmp=disconnected

core: expose PrivateTmp=disconnected

As discussed in https://github.com/systemd/systemd/pull/32724#discussion_r1638963071

I don't find the opposite reasoning particularly convincing.
We have ProtectHome=tmpfs and friends, and those can be
pretty much trivially implemented through TemporaryFileSystem=
too. The new logic brings many benefits, and is completely generic,
hence I see no reason not to expose it. We can even get more tests
for the code path if we make it public.