libstdc++: Fix socket option classes

libstdc++: Fix socket option classes

This fixes some flaws in the socket option types defined in
net::socket_base:

- 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.

Also:

- 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/bits/net.h (__socket_base): Add
	bool template parameter to allow BooleanSocketOption and
	IntegerSocketOption to have different __socket_base<int>
	base classes.
	(__socket_base<bool>): Adjust base class.
	(__socket_base<int>): Add partial specialization.
	(__socket_crtp::operator=(_Tp)): Add noexcept-specifier.
	* include/experimental/socket (socket_base::broadcast)
	(socket_base::debug, socket_base::do_not_route)
	(socket_base::keep_alive, socket_base::linger)
	(socket_base::out_of_band_inline)
	(socket_base::receive_buffer_size)
	(socket_base::receive_low_watermark)
	(socket_base::reuse_address, socket_base::send_buffer_size)
	(socket_base::send_low_watermark): Add using-declaration for
	__socket_crtp::operator=(_Tp).
	* testsuite/experimental/net/socket/socket_base.cc: Check
	properties of socket option types.