Clear J! passwd after change if nullpassword option is set

[cheesegrits]

Without this feature, if a user changes their password in J!, it gets written to the J! users table, and subsequent authentication then no longer happens against LDAP. Then if the LDAP user is removed, they can still log in to J!, and password changes in LDAP don't get used in J!. So we need to make sure the password is nulled in J! after a password change.

I know this PR is unlikely to get merged, as the author has moved on, but I thought I'd submit it anyway, in case anyone else needs this feature, and can merge it to their own fork.