auth: Fix handling passdbs with identical driver/args but different mechanisms/userna...

auth: Fix handling passdbs with identical driver/args but different mechanisms/username_filter

The passdb was wrongly deduplicated in this situation, causing wrong
mechanisms or username_filter setting to be used. This would be a rather
unlikely configuration though.

Fixed by moving mechanisms and username_filter from struct passdb_module
to struct auth_passdb, which is where they should have been in the first
place.

Gbp-Pq: Name auth-Fix-handling-passdbs-with-identical-driver-args-but-.patch