libstdc++: Better preprocessor conditions in net::ip [PR 100286]

libstdc++: Better preprocessor conditions in net::ip [PR 100286]

This improves the use of preprocessor conditionas to enable/disable
members of namespace net::ip according to what is supported by the
target. This fixes PR 100286 by ensuring that the to_string member
functions are always defined for the address_v4 and address_v6 classes.
On the other hand, the IP protocol classes and internet socket option
classes aren't useful at all if the corresponding constants (such as
IPPROTO_TCP or IPV6_MULTICAST_HOPS) aren't define. So those types are
not defined at all if they can't be used.

The net/internet/socket/opt.cc test uses __has_include to check whether
or not to expect the types to be available.

libstdc++-v3/ChangeLog:

	PR libstdc++/100286
	* include/experimental/internet (resolver_errc, resolver_category())
	(make_error_code, make_error_condition): Define unconditionally,
	only make enumerators and use of gai_strerror depend on the
	availability of <netdb.h>.
	(address_v4::to_string): Use correct constant for string length.
	(address_v4::to_string, address_v6::to_string): Define
	unconditionally, throw if unsupported.
	(make_address_v4, make_address_v6): Define unconditionally.
	Return an error if unsupported.
	(tcp, udp, v6_only, unicast::hops, multicast::*): Define
	conditionally,
	* testsuite/experimental/net/internet/socket/opt.cc: Check for
	<netinet/in.h> and <netinet/tcp.h> before using types from
	namespace net::ip.