s3:smbd:password_in_history: treat entry with 0 salt as 0 + plain nt hash

s3:smbd:password_in_history: treat entry with 0 salt as 0 + plain nt hash

This is to introduce a new format of the password history, maintaining backwards
compatibility: The old format was 16 byte hash + 16 byte md5(salt + nt hash).
The new format is 16 zero bytes and 16 bytes nt hash.

This will allow us to respect the last X entries of the nt password history
when deciding whether to increment the bad password count.

This is part of the fix for bug #4347 .

Michael