Check for availability of POSIX sockatmark before using it.

Rename _S_ntoh overloads that are ambiguous when passed an integral type
that is neither uint16_t nor uint32_t.

	PR libstdc++/89460
	* configure.ac: Check for sockatmark.
	* crossconfig.m4: Check for sockatmark.
	* config.h.in: Regenerate.
	* configure: Regenerate.
	* include/experimental/internet (address_v4::_S_hton): Rename
	overloaded functions to _S_hton_16 and _S_ntoh_16.
	(address_v4::_S_ntoh): Rename to _S_ntoh_16 and _S_ntoh_32.
	(basic_endpoint): Adjust calls to _S_hton and _S_ntoh.
	* include/experimental/socket (basic_socket::at_mark): Check
	_GLIBCXX_HAVE_SOCKATMARK.

From-SVN: r269588