site.c: Prepare for adding more MSG3 variants.

site.c: Prepare for adding more MSG3 variants.

  * Introduce a macro listing the known MSG3 variants.  Use this in
    `type_is_msg34' and `site_incoming', and in the `process_msg3'
    molly-guard.

  * Break out MSG3-ish label minor numbers and analyse them using the
    sensible ordering, in `generate_msg' and `unpick_msg'.

  * Have `check_msg' fall back to trusting `process_msg3' for all
    MSG3-ish messages.  (It already has a more vicious molly-guard
    anyway.)

  * Reformat the decision tree in `generate_msg3' so that adding more
    branches is cleaner.

Signed-off-by: Mark Wooding <mdw@distorted.org.uk>