Ethernet: Allow specifying non-standard Ethernet header length

Ethernet: Allow specifying non-standard Ethernet header length

A fair number of proprietary Ethernet switch tagging protocols, such as
Broadcom tags for instance, will place their tag between the MAC SA and
the Type/Length field.

Move the body of ether_print() into ether_print_hdr_len() and specify
the Ethernet header length as an argument to that function.
ether_print() calls ether_print_hdr_len() with a standard Ethernet
header lenght of 14 bytes, while other callers could specify an
arbitrary length. We still assume that the first Length/Type field to
parse is located 2 bytes before the end of that Ethernet header length.

This will be used in a subsequent commit to parse Broadcom tags.