Base class of all password handlers. Default behaviour is no passwords,
so anyone can be anyone they like.
The methods of this class should be overridded by subclasses that want
to implement other password handling methods.
If the $oldPassU is undef, it will try to add the user, failing
if they are already there.
If the $oldPassU matches matches the login's password, then it will
replace it with $newPassU.
If $oldPassU is not correct and not 1, will return 0.
If $oldPassU is 1, will force the change irrespective of
the existing password, adding the user if necessary.
Otherwise returns 1 on success, undef on failure.
Will return an encrypted password. Repeated calls
to encrypt with the same user/passU will return the same passE.
However if the passU is changed, and subsequently changed back
to the old user/passU pair, then the old passE is no longer valid.
If $fresh is true, then a new password not based on any pre-existing
salt will be used. Set this if you are generating a completely
new password.
