(for 4.9.3) libdnet has bugs, do not use it.

(for 4.9.3) libdnet has bugs, do not use it.

The only function tcpdump used in libdnet was dnet_htoa(), which tries
to translate a binary DECnet address to a nodename through a lookup in
/etc/decnet.conf. The translation is slow and has a bug, so stop using
the function and remove the dependency on libdnet.

This makes tcpdump always print DECnet addresses in numeric format, if
anybody needs the translation back they are welcome to fix libdnet or
(more realistically) add an implementation of dnet_htoa() to the tcpdump
source code and use it.