walk: adding functions for header traversing
walk: keep track of header size and validate caplen
capwalk: adding tool for dumping headers

Preferably used with --frame-num
capwalk: show capheader fields
format: use header_walk
format: adding utility function for name lookups
format: remove old function
ethernet: initial implementation
ethernet: reusable function

for usage with vlan
ip: initial implementation
icmp: initial implementation
mpls: initial implementation
tcp: initial implementation
udp: initial implementation
vlan: initial implementation
arp: initial implementation
cdp: initial implementation
gre: initial implementation
Stp trace
stp: initial implementation