shared/idmap: Fix various issues
shared/idmap: Fix various issues

 - Restore our old default map for when no shadow is found
 - Fix the logic that determines whether a map is valid or not to work
   with hybrid maps (entries that are listed with Isuid and Isgid both true).

Signed-off-by: Stéphane Graber <089afc6d81f66f1168a9849e15660feae286e024@ubuntu.com>

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