vmd(8): skip inspecting non-udp packets on local interfaces

vmd(8): skip inspecting non-udp packets on local interfaces

reyk@ imported packet.c from (I think) dhclient(8). In dhclient,
the packets are already filtered down to just udp. This change has
vmd skip a non-udp ip packet sooner when it's trying to determine
if it is a dhcp/bootp packet that needs processing.

The counters (seen, bad checksums, etc) for ip and udp packets are
also removed as they aren't used by vmd.

This only affects "local" network interfaces for vmd.

OK mlarkin@