What I've missed there was a description of "guaranteed contiguous
  data". The problem is that one function can make part of the mbuf chanin
  contiguous (like m_pulldown or one of the new functions), but a
  subsequent call can fragment the chain again. To prevent this, I have
  introduced the notion of "guaranteed contiguous data". If a function
  makes a range of a mbuf chain contiguous, it also marks it as a
  guaranteed contiguous region and subsequent calls to mbuf rearranging
  functions must keep it contiguous.

If it's not safe for a caller to assume it's still true, then the word
guaranteed shouldn't be used.  What you're describing is more like an
efficiency hint.  This also seems complicated, and if a caller can't
safely omit checks, I'm not sure how much is gained.