WB: Modernize packet parsing style.

WB: Modernize packet parsing style.

Enable ND_LONGJMP_FROM_TCHECK. Remove ND_TTEST*() instances that were
excess and convert the rest to ND_TCHECK*() with comments. Do not guard
ndo_snapend directly. Make sure that functions specific to message types
do not return on truncated data and return -1 when they have looked deep
enough to tell an invalid packet. In wb_print() deduplicate some code and
report invalid packets as invalid instead of truncated, also consider an
unknown packet type invalid. Update a test.