libuser: default user password

From the Red Hat advisory:

It was discovered that libuser did not set the password entry correctly when creating LDAP (Lightweight Directory Access Protocol) users. If an administrator did not assign a password to an LDAP based user account, either at account creation with luseradd, or with lpasswd after account creation, an attacker could use this flaw to log into that account with a default password string that should have been rejected. (CVE-2011-0002)