libstdc++: Minor refactoring in <experimental/internet>

libstdc++: Minor refactoring in <experimental/internet>

libstdc++-v3/ChangeLog:

	* include/experimental/internet (address_v6::bytes_type): Adjust
	formatting.
	(basic_endpoint): Define _M_is_v6() to put all checks for
	AF_INET6 in one place.
	(basic_endpoint::resize): Simplify.
	(operator==(const tcp&, const tcp&)): Add constexpr and noexcept.
	(operator!=(const tcp&, const tcp&)): Likewise.
	(operator==(const udp&, const udp&)): Likewise.
	(operator!=(const udp&, const udp&)): Likewise.
	* testsuite/experimental/net/internet/tcp.cc: New test.
	* testsuite/experimental/net/internet/udp.cc: New test.