Iterator: merge several changes from trunk.
- Update iterator_facade test for #1019
(header change already merged).
- Category of each iterator is reduced to a known category before we try to
find a minimum. Fixes #1517.
- `function_input_iterator` from Dean Michael Berris. Fixes #2893
- Fix typo in `boost/iterator.hpp`. Fixes #3434.
- Always include `add_reference` header in iterator adaptor header.
Did not merge changes for #1427.
[SVN r70709]
Daniel James