Further cleanups.

Further cleanups.

Use ND_TCHECK() rather than home-brew bounds checks.  Do simpler length
checks.

Let i be the length of the actual remaining packet data; use ND_TCHECK()
inside loops that iterate over the remaining data.

Let the printers for particular message types cast the raw data pointer
to a pointer of the appropriate type, rather than passing two pointers,
with different types, to the same data.