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.

src/core/unit.c

@@ -4131,6 +4131,9 @@ int unit_patch_contexts(Unit *u) {
                         ec->no_new_privileges = true;
                         ec->restrict_suid_sgid = true;
                 }
+
+                for (ExecDirectoryType dt = 0; dt < _EXEC_DIRECTORY_TYPE_MAX; dt++)
+                        exec_directory_sort(ec->directories + dt);
         }
 
         cc = unit_get_cgroup_context(u);