Fix HO_LEN for FreeBSD HEAD

John-Mark Gurney says:
  FreeBSD a couple years ago joined the Linux is making ip_len network
  byte order instead of host order (in
  https://svnweb.freebsd.org/changeset/base/r241913)...  I've attached
  a patch which finds the correct version of FreeBSD, and defines HO_LEN
  as necessary...

  With this patch, tcpcryptd is working on FreeBSD-HEAD...