[Swan-dev] prefix for C identifiers: "v2_" is better than "IKEv2_" or "ikev2_"

[D. Hugh Redelmeier]

I've introduced a new function, with a prefixed name.  I chose to break 
new ground by naming it "v2_build_id_payload".  I think that this is 
better than "ikev2_build_id_payload".

- adding IKE is redundant.  We know that we're dealing with IKE.  There is 
  no IPv2, for example.

- names should be easily distinguished.  Making the similar parts longer 
  makes this cognitively harder

- the standard for punch cards is no longer subject to revision so we're 
  stuck with 80 columns.  Let's not waste them

I've said this before.  Now I've acted.

Of course others may disagree.  If so, we can decide to rename it.

At some mythical time, when nothing is in flight, I'd love to rename all 
our identifiers to conform with this new convention.

It would perhaps be good to rename v1-only functions to have such a "v1" 
prefix.  But perhaps we'll ditch v1 support instead.