NetFlow: Modernize packet parsing style. [skip ci]

NetFlow: Modernize packet parsing style. [skip ci]

Enable ND_LONGJMP_FROM_TCHECK. Remove one redundant ND_TCHECK_SIZE()
instance and place the remaining ones after the code that prints
individual structure fields, so it can print incomplete structures. Lose
intermediate snprintf() calls and associated props. Lose trailing spaces
in v1 and v5 output.