libstdc++: Better requirements checking in Networking TS

Define concepts and traits for checking type requirements.

	* include/experimental/bits/net.h (__endpoint, __protocol)
	(__acceptable_protocol, __inet_protocol): New concepts.
	(__detail::__is_endpoint): Move trait from <experimental/socket>.
	(__is_protocol, __is_acceptable_protocol, __is_inet_protocol): New
	traits.
	(__endpoint, __protocol, __acceptable_protocol): New variable
	templates.
	* include/experimental/socket (__is_endpoint): Move to net.h header.
	(basic_socket, basic_socket_acceptor): Check requirements.