core: make StateDirectory= or friends works with DynamicUser= and RootDirectory=...

core: make StateDirectory= or friends works with DynamicUser= and RootDirectory=/RootImage=

The symbolic links to private directories specified by StateDirectory=
or its friends are created on the host. So, when DynamicUser= and
RootDirectory=/RootImage= are set, then the executed process cannot
access private directory.
This makes the private directories are mounted on the non-private place
when both DynamicUser= and RootDirectory=/RootImage= are set.

Fixes #8965.