Add std::copy_n istreambuf_iterator specialization

Add std::copy_n istreambuf_iterator specialization

	* include/bits/stl_algo.h
	(__copy_n_a(_IIte, _Size, _OIte)): New.
	(__copy_n_a(istreambuf_iterator<>, _Size, _CharT*)): New declaration.
	(__copy_n(_IIte, _Size, _OIte, input_iterator_tag)): Adapt to use
	latter.
	* include/bits/streambuf_iterator.h (istreambuf_iterator<>): Declare
	std::__copy_n_a friend.
	(__copy_n_a(istreambuf_iterator<>, _Size, _CharT*)): New.
	* testsuite/25_algorithms/copy_n/istreambuf_iterator/1.cc: New.
	* testsuite/25_algorithms/copy_n/istreambuf_iterator/1_neg.cc: New.
	* testsuite/25_algorithms/copy_n/istreambuf_iterator/2_neg.cc: New.

From-SVN: r276638