client: Adapted FuseAdapter to new design of UserMapping classes.

Slightly refactored the design: SystemUserMapping contains less pure virtual methods now and implements these on its own instead.

Because the FuseAdapter several additional methods which cannot be generalized, it has SystemUserMappingUnix as member and not the interface SystemUserMapping.

git-svn-id: https://xtreemfs.googlecode.com/svn/trunk@3596 f8528748-c2c0-11dd-ba5c-d3b7c2b3b534