Implement lowering 'ether broadcast'

According to the BPF syntax document, ether is optional, so 'broadcast'
is the same as 'ether broadcast'.
Implement lowering 'ether multicast'

According to the BPF syntax document, ether is optional, so 'multicast'
is the same as 'ether multicast'.
Implement lowering 'ether proto <proto>'

protos: ip, ip6, arp, rarp, atalk, aarp, decnet, sca, lat, mopdl, moprc, iso,
 stp, ipx, or netbeui. These identifiers are also keywords.
Add doc examples for 'ether multicast', 'ether broadcast' and 'ether proto <proto>'.
Merge pull request #55 from Igalia/low-ether

Implement several ethernet modifiers