r22020: Make it more clear that both the vuser struct and it's contents are

r22020: Make it more clear that both the vuser struct and it's contents are
talloc_free()'ed at the end of a session.

Rework the passwd cache code to use talloc_unlink and
talloc_reference, to more carefully manage the cache.

Andrew Bartlett
(This used to be commit e3e0ec25e67308de314aa61852905ee42aa2c8fe)