Add this suggestion to a batch that can be applied as a single commit.
  This suggestion is invalid because no changes were made to the code.
  Suggestions cannot be applied while the pull request is closed.
  Suggestions cannot be applied while viewing a subset of changes.
  Only one suggestion per line can be applied in a batch.
  Add this suggestion to a batch that can be applied as a single commit.
  Applying suggestions on deleted lines is not supported.
  You must change the existing code in this line in order to create a valid suggestion.
  Outdated suggestions cannot be applied.
  This suggestion has been applied or marked resolved.
  Suggestions cannot be applied from pending reviews.
  Suggestions cannot be applied on multi-line comments.
  Suggestions cannot be applied while the pull request is queued to merge.
  Suggestion cannot be applied right now. Please check back later.
  
    
  
    
What does this PR do
Adds a raw cmsg access api to the CmsgIterator. The goal is to bypass ControlMessageOwned which is marked non_exhaustive.
The SCTP messages I'm currently parsing hit the catchall ControlMessageOwned::Uknown, but my code would break if nix changes what the Unknown variant covers in the future, hence why I'm looking to access the messages raw.
I don't know enough about *nix to know if it's safe to return &'a references to the cmsg header and value so instead I borrowed from self, returning 1 control message at a time.
The safety of the unsafe blocks I believe are exactly the same as what I copied from ControlMessageOwned::decode_from.
Checklist:
CONTRIBUTING.md