hurd: Fix bits/socket.h conformity

	* sysdeps/mach/hurd/bits/socket.h: Include <bits/wordsize.h> instead
	of <limits.h>
	(__need_NULL): Do not define.
	(__ss_aligntype): Use __WORDSIZE instead of ULONG_MAX to determine
	alignment.
	[!__USE_MISC] (pseudo_AF_XTP, pseudo_AF_RTIP, pseudo_AF_PIP,
	CMGROUP_MAX, cmsgcred): Do not define.
	(CMSG_FIRSTHDR, __cmsg_nxthdr): Use (struct cmsghdr *) 0 instead of
	NULL.
	* bits/socket.h: Likewise.