Adding pwman into PSU::get and fixing usable pw check

While troubleshooting another issue today, I discovered that we did not have a shortcut for pwman in PSU.php This caused some alarm as login_hacks.php uses this call, so I am not sure if we have been in a broken state for some time, but I went ahead and added it for future expiration logic.

Also, I fixed a logic error in the PasswordManager when checking against old passwords. Had the check looking the wrong way on the activity_date.