MFstable/10 r228371:

This missing MFC fixes PR 199308

r228371 (by jhb):

• Add a test for PR 151758.
• While here, make this compile and work on non-i386:
  • Use CMSG_SPACE(), CMSG_LEN(), and CMSG_FIRSTHDR() instead of ignoring padding between 'struct cmsghdr' and control message payloads.
  • Don't initialize the control message before calling recvmsg(). Instead, check that we get a valid control message on return from recvmsg().
• Use errx() instead of err() for some errors that don't report failures that set errno.

Requested by: kib (1)