`od_hex_dump()` is called if `hdr_len < pkt->size` to print the rest
after `hdr_len` of `pkt`. So if we just leave `hdr_len = 0` instead of
calling `od_hex_dump()` for every other NETTYPE, we achieve the same
effect.
As it is more effective (and already done in some cases) to re-set
`hdr_len` when the header was printed, we initialize `hdr_len` first
with 0 now.
Currently, `gnrc_pktdump` only prints the header part of a snip.
However, if the snip wasn't parsed yet by the corresponding GNRC
module (or the module doesn't exist because the node is e.g. just a
forwarder), additional data might not be printed.
This makes it hard to analyze the data properly (sometimes you not only
want to know where the IPv6 packet is supposed to go, you also want to
know what's in it). So this just prints the rest of the snip as a hex
dump.
