Decouple pap & chap output routines from the corresponding

Decouple pap & chap output routines from the corresponding
input routines and take advantage of the new init/continue
interface in libradius.  This allows a timely response on
other links in an MP setup while RADIUS requests are in
progress as well as the ability to handle other data from
the peer in parallel.  It should also make the future addition
of PAM support trivial.

While I'm in there, validate pap & chap header IDs if
``idcheck'' is enabled (the default) for other FSM packet
types.

NOTE: This involved integrating the generation of chap
      challenges and the validation of chap responses
      (and commenting what's going on in those routines).
      I currently have no way of testing ppps ability
      to respond to M$Chap CHALLENGEs correctly, so if
      someone could do the honours, it'd be much
      appreciated (it *looks* ok!).

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