Clean up ISAKMP dissection, use nd_ types and EXTRACT_ macros.

Clean up ISAKMP dissection, use nd_ types and EXTRACT_ macros.

Add some RFC numbers.

Structures with nd_ types work well if you overlay them on top of the
packet buffer, but not as well with an on-the-stack structure into which
you copy from the packet, which is ugly if you do *both* with the same
structure.  Use overlaying uniformly.

Add EXTRACT_ macros as necessary for the nd_ types.

Make stuff unsigned if it's appropriate.

Use unsigned formats for unsigned values.

A number of routines are passed the length of a payload, so they don't
have to re-fetch that length themselves.  That length has been
pre-checked to make sure it's big enough for the payload header; note
that in comments.