Issue description:

I was trying to use passwd_s() for various password change operation, what I observed is there is no way to extract the system generated password when we do not provide new password. I checked the logs it does generate the random system password.

Steps to reproduce:

passwd_s(USER_DN, OLD_PASSWORD, None)

Operating system:

Fedora-28

Python version:

3.6.6

python-ldap version:

2.4.46-3.fc28