r1388: Adding password history code for ldap backend, based on a patch from

r1388: Adding password history code for ldap backend, based on a patch from
"Jianliang Lu" <j.lu@tiesse.com>. Multi-string attribute changed to
linearised pstring due to ordering issues. A few other changes to
fix race conditions. I will add the tdb backend code next. This code
compiles but has not yet been tested with password history policy
set to greater than zero. Targeted for 3.0.6.
Jeremy.