new auth capability 'logout'

This patch implements what Adrian's patch "Hide logout button if auth
backend cannot logout" intended to do.

The 'logoff' capability was used to decide if a special method called
$auth->logOff() should be called when the user logs out, not if the
backend supports logouts at all. This was a superflous capability since
an empty logOff() method is implemented in the base class anyway - it
doesn't hurt to always call the method.

The 'logoff' capability is now deprecated. Backends who want to do
actions on logout simply need to overwrite logOff().

A new capability 'logout' was added which defaults to true. Backends
that can't logoff the user (eg. because they use some automatic
login/logoff mechanism) can set this to false.

Probably makes sense to add a 'login' capability as well...