Define std::sample for C++17

Define std::sample for C++17

	* doc/xml/manual/status_cxx2017.xml: Add std::sample status.
	* doc/html/*: Regenerate.
	* include/experimental/algorithm (__sample): Move to bits/stl_algo.h
	and into namespace std.
	* include/bits/stl_algo.h (__sample): Define here. Fix invalid use
	of input iterator. Defend against overloaded comma operator.
	(sample): Define for C++17.
	* testsuite/25_algorithms/sample/1.cc: New test.

From-SVN: r241062