libstdc++: Add std::advance overload for ostreambuf_iterator

Implement std::advance overload for ostreambuf_iterator using 
basic_streambuf
pubseekof.

libstdc++-v3/ChangeLog:

         * include/bits/streambuf_iterator.h (ostreambuf_iterator): Add
         std::advance friend declaration.
         (advance(ostreambuf_iterator<>&, _Distance)): New.
         * testsuite/25_algorithms/advance/ostreambuf_iterator/char/1.cc:
         New test.
         * 
testsuite/25_algorithms/advance/ostreambuf_iterator/char/1_neg.cc:
         New test.
         * testsuite/25_algorithms/advance/ostreambuf_iterator/char/2.cc:
         New test.
         * 
testsuite/25_algorithms/advance/ostreambuf_iterator/char/2_neg.cc:
         New test.

Tested under Linux x85_64.

Ok to commit ?

François