User mapping is the process by which TWiki maps from a username (a login name)
to a display name and back. It is also where groups are maintained.
The BaseMapper provides support for a small number of predefined users.
No registration - this is a read only usermapper. It uses the mapper
prefix 'BaseUserMapping_'.
Users
TWikiAdmin - uses the password that was set in Configure (IF its not null)
Convert a login name to the corresponding canonical user name. The
canonical name can be any string of 7-bit alphanumeric and underscore
characters, and must correspond 1:1 to the login name.
(undef on failure)
If the $oldPassU matches matches the user'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.
returns a string indicating the error that happened in the password handlers
TODO: these delayed error's should be replaced with Exceptions.
returns undef if no error