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

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.