func CmsgSpace(datalen int) int {
	return cmsgAlignOf(SizeofCmsghdr) + cmsgAlignOf(datalen)
}
CmsgSpace returns the number of bytes an ancillary element with
payload of the passed data length occupies.
package unix
import "unsafe"
func cmsgAlignOf(salen int) int {
	salign := sizeofPtr
	if darwin64Bit || dragonfly64Bit {
		salign = 4
	}
	return (salen + salign - 1) & ^(salign - 1)
}
func CmsgLen(datalen int) int {
	return cmsgAlignOf(SizeofCmsghdr) + datalen
}