lwres: Modernize packet parsing style.

lwres: Modernize packet parsing style.

Enable ND_LONGJMP_FROM_TCHECK. Do not return on truncation. Let the
standard macro guard the snapshot end. Add integers instead of
subtracting pointers where possible. Make functions that now never return
a negative value unsigned and lose respective checks at the calling end.
Return -1 to tell an invalid packet. In lwres_printname() just use
nd_printn(). In lwres_print() add and use a standard invalid section.