Mention that OSL_RBTREE implies OSL_UNIQUE.

Also print a warning when a table is opened or created which contains
a column description with OSL_RBTREE set but OSL_UNIQUE unset. Future
versions of osl might reject such table descriptions. But in order
to not break existing applications, we can not make such a change
without a deprecation period. So a warning has to suffice for now.