r12936: Fix bug #3264.
If we detect a leading 'idmap_' for the idmap backend setting we strip
this and inform about the deprecated config with DEBUG 0.
I'm not sure if we should set a TTL of one year or five additional
releases from now for this code.
This is required for the changes Günther made as the first step to solve
bug #3264. With this fix we can even run with an old config. This is
very important as we else will break existing configurations with an
update.
(This used to be commit
34c7d8c0694369760340843318f873e26546cb2e)