use the tevent nesting code to avoid the uid problem in the VFS

use the tevent nesting code to avoid the uid problem in the VFS
backend

The vfs_unixuid module changes the uid of the process when executing
operations on behalf of the user. Within the VFS backend we may rely
on semi-async calls, such as winbind calls, which will call the event
loop again. To cope with this we need to ensure that while inside
those calls we revert the uid to root, then revert back to the
connected user when we have finished with the semi-async calls.