GeoNet: Modernize packet parsing style.

GeoNet: Modernize packet parsing style.

Enable ND_LONGJMP_FROM_TCHECK, lose a few single-use variables, print
one field at a time when possible and remove TCHECK*() instances that
are redundant. The only value print_long_pos_vector() could return was
0, so just make it void. In geonet_print() use nd_print_invalid() and
print the reason for declaring a packet invalid. Update comments and a
test.