Override the non-standard-dir-perm Lintian flag for /etc/backup.d, documenting...

Override the non-standard-dir-perm Lintian flag for /etc/backup.d, documenting why it is shipped this way.

debian/backupninja.lintian-overrides

+# Policy §10.9 wants 0755 or 2770 but since secrets could be stored there
+# we want this only accessed by root, and optionally by the "admingroup"
+# defined in backupninja.conf
+non-standard-dir-perm etc/backup.d/ 2770 != 0755