[boost] [filesystem] directory iterator operator++ patch

Beman, the attached patch enables the operator++(int) in directory_iterator. To summarize our discussion with Dave: 1. New single pass iterator requirements did not meants to outlaw returning proxy from operator++(int) 2. Even if proxy is automatically generated by iterator_facade, this might take some time to be implemented. In fact, I no longer believe it's really desired. So, implementing operator++(int) is directory_iterator makes sense. The attached patch basically uncomments your code, adds one using declaration to bring preincrement operator++ into the scope and changes the tests. Okay to commit? - Volodya