Usability issue: boost::tribool::value_t is public

The tribool interface is not very user-friendly, I saw more than once following error:

assignment:
````
boost::tribool v1 = boost::tribool::false_value;
boost::tribool v2 = boost::tribool::true_value;
boost::tribool v3 = boost::tribool::indeterminate_value;
````

comparison:

````
v1 == boost::tribool::false_value;
v2 == boost::tribool::true_value;
v3 == boost::tribool::indeterminate_value;
````

The issue is frequent because of auto completion.
Since `operator==`, `operator safe_bool` and the function `indeterminate` it is possible to query the status of `boost::tribool`, I do not think there are good reasons for the `value_t` to be public.

If `boost::tribool::value_t` is changed to private, some function like `indeterminate` and `operator!` needs to be made friends, but otherwise there should be no other issues.