This changes the way we do LDAP updates. We don't use LDAP_MOD_MODIFY

This changes the way we do LDAP updates. We don't use LDAP_MOD_MODIFY
anymore, but instead look at what is currently stored in the
database. Then we explicitly delete the existing attribute and add the
new value if it is not NULL or "". This way we can handle appearing
and disappearing attributes quite nicely.

This currently breaks pdbedit -o, as this does not set the CHANGED
flag on the SAM_ACCOUNT.

Jelmer suggested that we set all the fields on CHANGED in
context_add_sam_account. This sounds not too unreasonable.

Volker