core: add ExtensionDirectories= setting

Add a new setting that follows the same principle and implementation
as ExtensionImages, but using directories as sources.
It will be used to implement support for extending portable images
with directories, since portable services can already use a directory
as root.

src/test/test-namespace.c

@@ -207,6 +207,7 @@ TEST(protect_kernel_logs) {
                                     NULL,
                                     NULL,
                                     NULL,
+                                    NULL,
                                     NULL);
                 assert_se(r == 0);
 

src/test/test-ns.c

@@ -108,6 +108,7 @@ int main(int argc, char *argv[]) {
                             NULL,
                             NULL,
                             NULL,
+                            NULL,
                             NULL);
         if (r < 0) {
                 log_error_errno(r, "Failed to set up namespace: %m");