on LP64 systems (and probably the SPARC) since the __cmsg_alignbytes()
changes went in.
- Change file descriptor passing to use CMSG_DATA(), not (cm + 1). This
pretty much has to be done in order to make it work properly on LP64,
and considering that it's been broken this long...
- Use CMSG_SPACE() to determine the mbuf length needed for a given
control message, and CMSG_LEN() to stash in the cmsg_len member.