Also add some missing member functions, nodiscard attributes, and
noexcept-specifiers.

Backport from mainline
2020-05-21  Jonathan Wakely  <jwakely@redhat.com>

	* include/experimental/executor (use_future_t::use_future_t()): Fix
	incorrect noexcept-specifier.
	* include/experimental/internet (basic_resolver_results): Adjust
	whitespace.
	* include/experimental/socket (__basic_socket_impl::release): Add
	member function.
	(basic_socket(io_context&, const endpoint_type&)): Fix argument to
	target constructor.
	(basic_socket::release(), basic_socket::release(error_code&)): Add
	missing member functions.
	(basic_socket::is_open()): Add nodiscard attribute.
	(basic_socket::close(error_code&)): Pass argument to base function.
	(basic_socket_acceptor::release())
	(basic_socket_acceptor::release(error_code&)): Add missing member
	functions.
	(basic_socket_acceptor::is_open()): Add nodiscard attribute.
	(basic_socket_streambuf::error()): Add noexcept.
	(basic_socket_iostream::error()): Likewise.
	* testsuite/experimental/net/socket/basic_socket.cc: New test.