libstdc++: Fix internet socket option classes

libstdc++: Fix internet socket option classes

Similar to the previous commit, this fixes various problems with the
socket options classes in the <internet> header:

- The constructors were not noexcept.
- The __sockopt_base<T>::value() member function was present
  unconditionally (so was defined for socket_base::linger which is
  incorrect).
- The __socket_crtp<C, T>::operator=(T) assignment operator was not
  noexcept, and was hidden in the derived classes.
- The MulticastSocketOptions incorrectly used a union, incorrectly
  defined resize and const data() member functions, and their
  constructors were unimplemented.

Also, where appropriate:

- Use class instead of struct for the socket option types.
- Define the _S_level and _S_name constants as private.
- Declare the __socket_crtp base as a friend.

libstdc++-v3/ChangeLog:

	* include/experimental/internet (tcp::no_delay, v6_only)
	(unicast::hops, multicast::hops, multicast::enable_loopback):
	Change access of base class and static data members. Add
	using-declaration for __socket_crtp::operator=(_Tp).
	(multicast::__mcastopt): New type.
	(multicast::join_group, multicast::leave_group): Derive from
	__mcastopt for common implementation.
	* include/experimental/socket: Add comment.
	* testsuite/experimental/net/internet/socket/opt.cc: New test.
	* testsuite/experimental/net/socket/socket_base.cc: Check for
	protected constructor/destructor of socket_base. Check for
	explicit constructors of socket option classes.