Security context requires a Zope user, not a Plone member.

Fix for d7353810bf0bd9f95cfc3c71a30a15e2c8e5eec1 --- it introduces a memberdata instance into the security context, which breaks things. 

Calling members users when they're really (1) quite different things, and (2) both relevant in context, is needlessly confusing. 

The other methods like `getCurrentUserRights` are less harmful, since they don't work differently for members and users. For consistency we could deprecate them also.