s3:passdb: store the plain nt passwords hashes in history, not salted md5

s3:passdb: store the plain nt passwords hashes in history, not salted md5

This is in order to be able to do challenge response with the history,
so that this can be checked when an invalid password was entered:
If the given password is wrong but in the history, then the bad password
count should not be updated...

The "lucky" bit here is that the md5 has and the nt hash (md4) both are
16 bytes long.

This is part of the fix for bug #4347 .

Michael
(cherry picked from commit d909861c64cf874b1625039b0e1eace507a29b28)