core: add [State|Runtime|Cache|Logs]Directory symlink as second parameter

core: add [State|Runtime|Cache|Logs]Directory symlink as second parameter

When combined with a tmpfs on /run or /var/lib, allows to create
arbitrary and ephemeral symlinks for StateDirectory or RuntimeDirectory.
This is especially useful when sharing these directories between
different services, to make the same state/runtime directory 'backend'
appear as different names to each service, so that they can be added/removed
to a sharing agreement transparently, without code changes.

An example (simplified, but real) use case:

foo.service:
StateDirectory=foo

bar.service:
StateDirectory=bar

foo.service.d/shared.conf:
StateDirectory=
StateDirectory=shared:foo

bar.service.d/shared.conf:
StateDirectory=
StateDirectory=shared:bar

foo and bar use respectively /var/lib/foo and /var/lib/bar. Then
the orchestration layer decides to stop this sharing, the drop-in
can be removed. The services won't need any update and will keep
working and being able to store state, transparently.

To keep backward compatibility, new DBUS messages are added.