auc: fix several issues

 * don't segfault if server reply is incomplete
 * always request 'generic' image on x86/generic and x86/64
 * make SNAPSHOT branches sticky, eg. stay on 21.02-SNAPSHOT and do
   not 'upgrade' to 21.02.0-rc2.
 * output requested server URL when debugging

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
(cherry picked from commit 02bc1fc9)