importd: add support for downloading sysext/confext/portable images too

This adds "Ex" versions of all bus calls import implements, that make
two changes:

1. A "class" parameter is added that allows choosing between
   machine/sysext/confext/portable images to download. Depending on the
   chose class the target directory is selected differently (i.e. not
   just /var/lib/machines/, but alternatively /var/lib/portables/,
   /var/lib/extensions/, /var/lib/confexts/.

2. The boolean flags are replaced by a 64bit flags parameter.

src/import/pull-common.c

@@ -37,11 +37,9 @@ int pull_find_old_etags(
         int r;
 
         assert(url);
+        assert(image_root);
         assert(etags);
 
-        if (!image_root)
-                image_root = "/var/lib/machines";
-
         _cleanup_free_ char *escaped_url = xescape(url, FILENAME_ESCAPE);
         if (!escaped_url)
                 return -ENOMEM;
@@ -128,11 +126,9 @@ int pull_make_path(const char *url, const char *etag, const char *image_root, co
         char *path;
 
         assert(url);
+        assert(image_root);
         assert(ret);
 
-        if (!image_root)
-                image_root = "/var/lib/machines";
-
         escaped_url = xescape(url, FILENAME_ESCAPE);
         if (!escaped_url)
                 return -ENOMEM;