core: do not create symlink to private directory if parent already exists

core: do not create symlink to private directory if parent already exists

The very basic functinality of StateDirectory= or friends is creating
specified directories. That should work if one entry is a subdirectory
of another. However, it does not when combined with DynamicUser=yes.

To support such case, this adds ExecDirectoryItem.only_create flag, and
if it is set PID1 only create private directory, and not create the symlink
to the private directory.

Fixes #24783.