Assert restrictive permissions on the key database [CVE-2023-1672]

Since 11-2, the window for an attack was already pretty small, let's
close the remaining gap, and assert permissions when upgrading from an
earlier version.

debian/tang-common.postinst

@@ -33,8 +33,9 @@ configure)
     fi
 
     # assert db directory
-    mkdir -p "$DB_DIR"
+    mkdir -m0750 -p "$DB_DIR"
     chown _tang:_tang "$DB_DIR"
+    # assert restrictive permissions upon upgrade
     chmod 0750 "$DB_DIR"
 
     ;;