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/dbus-execute.c

@@ -3324,6 +3324,7 @@ int bus_exec_context_set_transient_property(
                                         if (r < 0)
                                                 return log_oom();
                                 }
+                                exec_directory_sort(d);
 
                                 joined = unit_concat_strv(l, UNIT_ESCAPE_SPECIFIERS);
                                 if (!joined)
@@ -3787,6 +3788,8 @@ int bus_exec_context_set_transient_property(
                 if (r < 0)
                         return r;
 
+                exec_directory_sort(directory);
+
                 r = sd_bus_message_exit_container(message);
                 if (r < 0)
                         return r;