MFC r225030:

MFC r225030:

 While not explicitly allowed by RFC 2460, in case there is no
 translation technology involved (and that section is suggested to
 be removed by Errata 2843), single packet fragments do not harm.

 There is another errata and further drafts under discussion to clarify
 on these kinds of packets.
 Meanwhile add a sysctl to allow disabling this behaviour again.
 We will treat single packet fragment (a fragment header added
 when not needed) as if there was no fragment header.

 Submitted by:	Matthew Luckie (mjl luckie.org.nz) (original version)
PR:		kern/145733

git-svn-id: svn://svn.freebsd.org/base/stable/8@231206 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f