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/core/execute.c

@@ -2065,6 +2065,9 @@ bool exec_needs_mount_namespace(
         if (context->n_extension_images > 0)
                 return true;
 
+        if (!strv_isempty(context->extension_directories))
+                return true;
+
         if (!IN_SET(context->mount_flags, 0, MS_SHARED))
                 return true;
 
@@ -3566,6 +3569,7 @@ static int apply_mount_namespace(
                             context->root_verity,
                             context->extension_images,
                             context->n_extension_images,
+                            context->extension_directories,
                             propagate_dir,
                             incoming_dir,
                             root_dir || root_image ? params->notify_socket : NULL,
@@ -5244,6 +5248,7 @@ void exec_context_done(ExecContext *c) {
         c->root_hash_sig_path = mfree(c->root_hash_sig_path);
         c->root_verity = mfree(c->root_verity);
         c->extension_images = mount_image_free_many(c->extension_images, &c->n_extension_images);
+        c->extension_directories = strv_free(c->extension_directories);
         c->tty_path = mfree(c->tty_path);
         c->syslog_identifier = mfree(c->syslog_identifier);
         c->user = mfree(c->user);
@@ -6120,6 +6125,8 @@ void exec_context_dump(const ExecContext *c, FILE* f, const char *prefix) {
                                 strempty(o->options));
                 fprintf(f, "\n");
         }
+
+        strv_dump(f, prefix, "ExtensionDirectories", c->extension_directories);
 }
 
 bool exec_context_maintains_privileges(const ExecContext *c) {