tree-wide: Get rid of prefix_roota() in favor of path_join()

We deprecated prefix_roota() in favor of chase() and path_join().
Let's finish the removal by replacing the few remaining call sites
with path_join().

src/network/generator/main.c

@@ -123,7 +123,9 @@ static int context_save(Context *context) {
         Link *link;
         int r;
 
-        const char *p = prefix_roota(arg_root, NETWORK_UNIT_DIRECTORY);
+        _cleanup_free_ char *p = path_join(arg_root, NETWORK_UNIT_DIRECTORY);
+        if (!p)
+                return log_oom();
 
         r = mkdir_p(p, 0755);
         if (r < 0)